Ok, now that I know it's an Intex I can help.

Now that the lines are plugged, pour the water out of the filter. Dry out the o-ring's slot, and the ring. If you have teflon o-ring lube, that's better.

Next you need to force the o-ring into its slot all the way. Don't use anything metal or you'll tear it--a thumb is best. You need to work the o-ring slowly into slot all the way around--it may be a little big (a real pain) so you'll have to sort of distibute it around the whole slot. Then you can put in the filter and put the cover back on, tightening it down. Then pull the plugs and see if it's sealed. Remember to put the spacer on the cartridge (if they still use them) and situate the cartridge correctly on the pipe at the bottom of the chamber.

At least I think that's how it all works--it's been several years since I opened my Intex pump--it's retired 4 years now.