Can’t access subdomains?!

I’m sure this is a total noob thing, but any help would be greatly appreciated.

I’m running WP 2.7 in my top level domain.
I’ve got a membership program (amember) installed in mydomain/member

But when I try to access the program at mydomain/member/ I get one of two things:

1.) If I have a custom permalink setting in WP I get my actual template, and a “not found” post.

2.) If I have the default permalink setting, I get a standard 404 error from the server.

So… How do I get at my membership software?!

Thanks for your help!

Best,
Chris