Thanks for your help, Daxhua! A member of another forum checked his XP3 for me - the groove faces the sides of the filter, while the ridge faces up/down. I tried it that way, with no leaks so far.
I really appreciate your advice for getting the air out of the canister. I contacted Planet Rena about that problem before and they were no help - they kept telling me I must have primed it incorrectly. I tried your suggestion this morning with great results. Plus, the filter seems to be pumping faster now that there's no air trapped inside.