With the release of SDK 2.0 I'm no longer able to build fresh C projects for the Windows Simulator with nmake, including the SDK provided examples, unless I create an empty "pdex.bin" or "main.lua" in the "Source" folder first. After that it's business as usual.
Thanks for the report, we've identified the issue and will have it fixed in the next release. In the meantime, it shouldn't hurt to keep a main.lua file in your Source folder.