Yup. I feel dumb.
If anyone else is having a similarly vapid moment. Just use the rails console to create the admin user:
➡ rails c
Loading development environment (Rails 3.0.0.beta3)
irb(main):001:0> admin = Admin.create! do |u|
irb(main):002:1* u.email="sample@sample.com"
irb(main):003:1> u.password = 'password'
irb(main):004:1> u.password_confirmation = 'password'
irb(main):005:1> end
That will do it. Now just visit your admin sign in path and sign in.