I am using Machinist 2 beta with Rails 3.1 and its working quite nicely. Though its beta, I've found it to be working quite well.
To do some manual "manufacturing", I use the console, but for that to work, I have to load the test helper:
Loading test environment (Rails 3.1.0.rc4) >> require "./test/test_helper.rb"
For what its worth, here's a sample of my blueprints.rb file, too:
1 require 'machinist/active_record' 2 3 Asset.blueprint do 4 name { "Photo Copier" } 5 end 6 7 BankAccount.blueprint do 8 name { "National Bank" } 9 number { 12345 } 10 end