1. Modify /app_data/users.xml adding (or updating) default profile for admin/admin:
2. Modify /web.config forcing IIS to restart application, any minor change to this file like adding or removing comment will do.
Now you should be able to login as admin/admin. That is, if you are using default XML membership provider. If you on SQL server, then instead of modifying file you would need to modify user record in DB.