Had the same issue. A right-click on the project -> Maven -> 'Reload project' may probably fix it.

For Intellij Idea: go to Maven(on right side) and press "Reimport All Maven Project".

Try running the following commands and examine the output:

$ mvn dependency:tree
$ mvn help:effective-pom

Look for commons-lang, maybe something will draw your attention like excludes or dependency overrides. Also, is:

$ mvn dependency:copy-dependencies

copying commons-lang JAR to your target?

If you can use Maven or Gradle, then you can include the library as a dependency:

$ tree
.
|-- pom.xml
`-- src
    |-- main
    |   `-- java
    |       `-- test
    |           `-- App.java
    `-- test
        `-- java
            `-- test
                `-- AppTest.java

7 directories, 3 files

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>myGroup</groupId>
  <artifactId>myArtifact</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>deleteme</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.apache.commons</groupId>
      <artifactId>commons-lang3</artifactId>
      <version>3.1</version>
    </dependency>
  </dependencies>
</project>

IntelliJ automatically adds the dependencies to the classpath.

If you open the pom.xml file as a new project in IntelliJ, the dependency will automatically be added to the classpath.

Edit

Otherwise, if you edit your project settings in IntelliJ, go to the modules section, and then the dependencies tab. You can add your dependencies there. See here.

You should add the unpacked jar file from commons-csv-1.6-bin.zip\commons-csv-1.6\commons-csv-1.6.jar into the Module Dependencies.

If using Gradle, add the library via build.gradle configuration.

This is the problem:

Copyimport org.apache.commons.math3;

That's trying to import a package - you can't do that. You have to either use a wildcard import:

Copyimport org.apache.commons.math3.*;

or import a specific type:

Copyimport org.apache.commons.math3.SomeTypeHere;

In your case it sounds like you actually want:

Copyimport org.apache.commons.math3.distribution.EnumeratedIntegerDistribution;

I've tried a sample class with just that import and the jar file downloaded from Apache, and it works fine.