Since you already know the formula, it should be easy enough to create a function to do the calculation for you.

Here, I've created a basic function to get you started. The function takes four arguments:

• frequencies: A vector of frequencies ("number" in your first example)
• intervals: A 2-row matrix with the same number of columns as the length of frequencies, with the first row being the lower class boundary, and the second row being the upper class boundary. Alternatively, "intervals" may be a column in your data.frame, and you may specify sep (and possibly, trim) to have the function automatically create the required matrix for you.
• sep: The separator character in your "intervals" column in your data.frame.
• trim: A regular expression of characters that need to be removed before trying to coerce to a numeric matrix. One pattern is built into the function: trim = "cut". This sets the regular expression pattern to remove (, ), [, and ] from the input.

Here's the function (with comments showing how I used your instructions to put it together):

GroupedMedian <- function(frequencies, intervals, sep = NULL, trim = NULL) {
  # If "sep" is specified, the function will try to create the 
  #   required "intervals" matrix. "trim" removes any unwanted 
  #   characters before attempting to convert the ranges to numeric.
  if (!is.null(sep)) {
    if (is.null(trim)) pattern <- ""
    else if (trim == "cut") pattern <- "\\[|\\]|\\(|\\)"
    else pattern <- trim
    intervals <- sapply(strsplit(gsub(pattern, "", intervals), sep), as.numeric)
  }

  Midpoints <- rowMeans(intervals)
  cf <- cumsum(frequencies)
  Midrow <- findInterval(max(cf)/2, cf) + 1
  L <- intervals[1, Midrow]      # lower class boundary of median class
  h <- diff(intervals[, Midrow]) # size of median class
  f <- frequencies[Midrow]       # frequency of median class
  cf2 <- cf[Midrow - 1]          # cumulative frequency class before median class
  n_2 <- max(cf)/2               # total observations divided by 2

  unname(L + (n_2 - cf2)/f * h)
}

Here's a sample data.frame to work with:

mydf <- structure(list(salary = c("1500-1600", "1600-1700", "1700-1800", 
    "1800-1900", "1900-2000", "2000-2100", "2100-2200", "2200-2300", 
    "2300-2400", "2400-2500"), number = c(110L, 180L, 320L, 460L, 
    850L, 250L, 130L, 70L, 20L, 10L)), .Names = c("salary", "number"), 
    class = "data.frame", row.names = c(NA, -10L))
mydf
#       salary number
# 1  1500-1600    110
# 2  1600-1700    180
# 3  1700-1800    320
# 4  1800-1900    460
# 5  1900-2000    850
# 6  2000-2100    250
# 7  2100-2200    130
# 8  2200-2300     70
# 9  2300-2400     20
# 10 2400-2500     10

Now, we can simply do:

GroupedMedian(mydf$number, mydf$salary, sep = "-")
# [1] 1915.294

Here's an example of the function in action on some made up data:

set.seed(1)
x <- sample(100, 100, replace = TRUE)
y <- data.frame(table(cut(x, 10)))
y
#           Var1 Freq
# 1   (1.9,11.7]    8
# 2  (11.7,21.5]    8
# 3  (21.5,31.4]    8
# 4  (31.4,41.2]   15
# 5    (41.2,51]   13
# 6    (51,60.8]    5
# 7  (60.8,70.6]   11
# 8  (70.6,80.5]   15
# 9  (80.5,90.3]   11
# 10  (90.3,100]    6

### Here's GroupedMedian's output on the grouped data.frame...
GroupedMedian(y$Freq, y$Var1, sep = ",", trim = "cut")
# [1] 49.49231

### ... and the output of median on the original vector
median(x)
# [1] 49.5

By the way, with the sample data that you provided, where I think there was a mistake in one of your ranges (all were separated by dashes except one, which was separated by a comma), since strsplit uses a regular expression by default to split on, you can use the function like this:

x<-c(110,180,320,460,850,250,130,70,20,10)
colnames<-c("numbers")
rownames<-c("[1500-1600]","(1600-1700]","(1700-1800]","(1800-1900]",
            "(1900-2000]"," (2000,2100]","(2100-2200]","(2200-2300]",
            "(2300-2400]","(2400-2500]")
y<-matrix(x,nrow=length(x),dimnames=list(rownames,colnames))
GroupedMedian(y[, "numbers"], rownames(y), sep="-|,", trim="cut")
# [1] 1915.294

This formula is the result of a linear interpolation, which identifies the median under the assumption that data are uniformly distributed within the median class.

To derive the formula, we can note that since $N/2$ is the number of observations below the median, then $N/2 - F_{m-1}$ is the number of observations that are within the median class and that are below the median ($F_{m-1}$ is the cumulative frequency of the interval below the median class, i.e. of all classes below the median class).

As a result, the fraction $\displaystyle\frac {N/2 - F_{m-1}}{f_m}$ (where $f_m$ is the frequency of the median class) represents the proportion of data values in the median class that are below the median.

Now if we assume that data are uniformly distributed (i.e., equally spaced) within the median class, multiplying the last fraction by $c$ (total width of the median class) we obtain the fraction of median class width corresponding to the position of the median. Adding the result to $L_m$ (lower limit of the median class), we get the final formula $\displaystyle L_m + \left [ \frac { \frac{N}{2} - F_{m-1} }{f_m} \right ] \cdot c$, which identifies the median.