We've got the details on how to do this below. However, there are currently a bunch of orc gibs in the tubes so it doesn't work right! We are getting this fixed!
1. Go to the old Robot Entertainment website: http://v2.robotentertainment.com
2. Log into that site. (Email and password).
3. Go to your profile and select the Connections tab.
It will take a little bit for the automated system to process the connection and send the achievement over to your Steam account.