ImageScience on Mac is not loaded

Question

hi, i'm a ruby-newbe trying to get attachment_fu working (with ImageScience as processor). After few days of exhausting battles I came to the point where (using console):

irb> require 'image_science'
true

so i'm assuming there are no problems with loading. however my application throws constantly:

Cannot load file -- image_science

Is there any kind of "classpath" I need to setup to make this image_science gem visible for ruby? I can't understand why it's perfectly available for irb and not for my rails application.

I'm using Ruby 1.9.1 and rails 3beta4 (the same problems occur on 2.3.8)