Looks like it works, but doesn't

I need help. I successfully installed wpmu on two different sites for my brother. Then I install it on a 3rd site for me, and it just doesn't work. The admin blog is up, but when I try to create a "test" blog, it looks likes it working, I receive the link to activate the blog, but the link to activate the blog doesn't work. Does this have to do with blog.domain.com vs. domain.com/blog ??? I've installed wpmu about 5 times now and it just doesn't work like it did on the prior 2 sites.