Use abi.encodePacked(x)

where x is the address. (Thanks @k06a)

In Solidity, an address is represented by a 20-byte value. You can convert a string that represents an Ethereum address to an address type using the address function. Here's an example:

function myFunction(string memory addressString) public {
  address myAddress = address(bytes20(bytes(addressString)));
  // Do something with myAddress
}

In this example, addressString is the string that represents an Ethereum address. The bytes(addressString) converts the string to a byte array, and bytes20 takes the first 20 bytes of that array. Finally, address casts the resulting 20-byte value as an Ethereum address.

Note that the input string must be a valid Ethereum address in order for this conversion to work properly. If the input string is not a valid Ethereum address, this code may result in an error.

Also, be aware that you may need to handle the possibility of invalid input strings in your contract code, either by validating the input before passing it to address, or by adding appropriate error handling code.

One issue with your code is that you are making a call, but your function is not constant and modifies the storage, you need to issue a standard transaction for that reason

CoinFlipper.deployed().then(function(instance) { 
    meta = instance; 
    return meta.check(acc, name, password); /// <--- without `call`
}).then(function (value) {
}); 

From oraclize contract

function parseAddr(string memory _a) internal pure returns (address _parsedAddress) {
    bytes memory tmp = bytes(_a);
    uint160 iaddr = 0;
    uint160 b1;
    uint160 b2;
    for (uint i = 2; i < 2 + 2 * 20; i += 2) {
        iaddr *= 256;
        b1 = uint160(uint8(tmp[i]));
        b2 = uint160(uint8(tmp[i + 1]));
        if ((b1 >= 97) && (b1 <= 102)) {
            b1 -= 87;
        } else if ((b1 >= 65) && (b1 <= 70)) {
            b1 -= 55;
        } else if ((b1 >= 48) && (b1 <= 57)) {
            b1 -= 48;
        }
        if ((b2 >= 97) && (b2 <= 102)) {
            b2 -= 87;
        } else if ((b2 >= 65) && (b2 <= 70)) {
            b2 -= 55;
        } else if ((b2 >= 48) && (b2 <= 57)) {
            b2 -= 48;
        }
        iaddr += (b1 * 16 + b2);
    }
    return address(iaddr);
}