The idea of the bullet workspace file is to compile bullet I think, but not start your own project in. Just start a brand new project for yourself, THEN include the three bullet projects files (not the .sln file but the .vcproj files), then set the include directory so it can find btBulletDynamicsCommon.h or btBullenCollisionCommon.h (or something very similar like that, see manual for the exact names).
And there are some other minor things you should set I think. Just follow the tutorial at 'create your own project from scratch' link on the wiki that I gave earlier, it worked fine for me.
just start reading from 'Create a new Visual Studio project'
Then see the Hello World tutorial on how to create your first very own bullet program that does something. From there on you can just continue working on your project using bullet.
edit: Sorry I read again and it seems like you are interested in making your own demo's, then it might be good to use the bullet workspace that is already there (But not sure). A drawback on using demo's that is that you ALSO have to learn to use the demo framework. Once you do that you can quickly make new demo's with lots of functionality fast, but my preference is always to start with nothing when i learn something new. If you just want to learn from scratch and you don't want to mess around with the demo framework yet, I suggest what I said above.