I ran Mod_RSSIM without configuring any COM ports, so it naturally told me it couldn't open one. Then, I installed com0com and went with the default virtual port names, but this didn't show up in the simulator, either. Assuming it looks for ports with standard names, I renamed the virtual ports to COM3 and COM4, but they still didn't appear. The com0com readme suggested installing "fake" ports in the Device Manager, so I did that for both COM3 and COM4, but the simulator still doesn't see them. The Termite serial terminal program can see them, but Mod_RSSIM cannot.
I'm running the latest version on Windows 7 Professional 64-bit.