ok some of my meshes are one sided, so i can see right through em. um... is that fixable or were the meshes just made that way?. second of all, when i try to change a mesh to a brush for that collision stuff, it crashes... lil help.
dunno whether meshes have a "twosided" preference, but materials have. so try setting the texes "twosided" option to "true" and it should work.