Want to Use my existing online ID with reinstalled game

I had to reinstall Steam on my macbook Pro and GSB is trying to create my online ID. I want to reuse my old online ID but it says it is in use (from my previous install). Is there a way I can recover this?

hi, if you email me (cliff @ positech dot co dot uk) with the serial number and username you had, I can reset it in the database so you can re-claim that username.