Membership plugin installed but now showing? HELP!

I just downloaded and installed the membership plugin, but it's not showing up in my admin at all, not even in the list of plugins (active or inactive).

I am attempting to add it to an existing site with many plugins (and built on Thesis) just as an FYI.

I'm on quite a tight deadline to get this up and running (this is the site so any help would be appreciatted. Thanks in advance.