How have you got this set up? Where is the Multiplexer2LEDs.cpp, should it not be in a tab or the library?
The error message talks about a loop function in this code, you can only have one loop function.
As the error is with the Multiplexer2LEDs.cpp file it is a little difficult trying to fault find on it as you haven't included it.
Try zipping up your project file and posting that.