Help using MUX in Logisim
I think mux databits needs to be set to 1. If you right click on the mux or your pins, you can change the number of databits in the lefthand attribute window. Also, on the mux, you probably don't need the enable input, so turn that off.

Your mux should have only 1 output line. Your control line is supposed to select which of your inputs will be connected to your single output line. Having 2 output lines doesn't fit the normal design of a 2-line to 1-line mux. 

You can assign wider paths to the mux inputs and outputs, but it doesn't appear to be what you are trying to do in this case.