Using iSim to simulate 16-bit CLA schematic on Xilinx, all inputs and outputs on the waveform are 'X'. How can I debug?
Don't assume it's good just because the professor gave it to you.  If you haven't learned this already, you will learn this at some point.  If you assume, you make an ass of u and me.  VERIFY YOUR ASSUMPTIONS.  To me, it looks like there may be some issues in the testbench.  

Two things to try: initialize the regs to zero by adding = 0 right before the semicolons (e.g. reg C0 = 0;).  Second thing to try is commenting out the ifdef/endif around the initial block (add // to the beginning of both lines).  

If those don't work, isolate it by commenting out the entire UUT block and try again.