What type of adhesive could I use to add pond rocks to my pond liner?
Don't glue anything to the liner itself? If you want to glue some rocks together for any reason, there is a silicone that is safe for aquariums that could work.
Dry stacking rocks or using flat/smooth ones to cover the bottom, is the usual way to accomplish coverage of the liner.
For general message board help, click the tab labeled "Tools," and choose "Help" from the dropdown menu.