There were a few posts a while ago about missing Prefabs...
We did try to put people on track, but when I was mucking around with my New cs map, I found that Hammer 3.5 doesn't have any Prefabs included.
If you have this problem, just copy the prefabs from 3.4 into your file structure for 3.5 or use 3.4
I only use 3.5 for model placement and the occasional LEAK trace. Mostly I use 3.4