Ahhh,
My apologies, I accidentally ignored that you were on EG3.
The "Assign Library ..." option is a feature of EG4.
It has been over 2 years since I last used EG3, so unfortunately, I cannot be of help.
I would strongly recommend, if I may, to upgrade to 4.1. It is a huge, HUGE, improvement over 3. ! HUGE ! Although, I realize this doesn't help it, it will, a lot. Did I say "HUGE"? enough?
In my opinion, the solution to your problem is to upgrade to EG4.x