I just downloaded the beta, and tried to install it. I get the initial form to enter in my database info. Once I hit install, I get a page that is completely unstyled, but says it has installed correctly - and provided url's for the site and for the admin.

Once I go anywhere else, I get this error:
Critical Error: Could not load class BIGTREE_CUSTOM_BASE_CLASS.

Any ideas? Would love to try the CMS, but this does not bode well.