Add these lines (if they're not there) to your /etc/slapt-get/slaptgetrc file
EDIT: Corrected the sources for ftp:
or simply open gslapt, go to Edit, Preferences, Sources, and where it says ADD write:
repeating for the other repositories that you want to have enabled.
It might be better to add "testing" as a source, but not enable it, unless you want a specific package in there, or if something you want to install has a dependency that is in the "testing" repository.