I'm just checking in here as another user with the problem. I started with the free scirra.com version and did some tutorial projects. I later installed the steam version and it wouldn't preview projects. However, if I loaded one of my previously saved projects (from the scirra.com version) it would export those and preview in browser. Very weird. After trying a few things unsuccessfully I deleted the steam version and went back to the original. It works fine.
In case it's a contributor to the problem, I am using an ATI/AMD graphics card. I doubt that is the issue though.