app[web.1]: /app/vendor/bundle/ruby/1.9.1/gems/mm-devise-1.3/lib/devise/orm/mongo_mapper.rb:24:in `module:Schema': no such file to load -- devise/schema (LoadError)
I think this gem needs to be reworked from the ground up. Look at the current devise mongoid integration and see if you can mimick it for Mongo Mapper. Devise has changed a lot since I designed this gem. Back then, devise didn't even come with mongoid support baked in as it does now (as far as I know).
If you look at my Devise 2.1 upgrade (see my fork) this should be fixed for you.
I didn't build from the group up, but I certainly looked at the mongoid integration to understand what had changed.
Pull request merged in fixes issue :)