ORIGINAL: carrellh
If you lubricate the stopper it should go in. Be sure the srew is loose so the stopper is not expanded at all.
what he said... i just put one together last night. definitely be sure the screw is loose... i cleaned up the edge of the tank so there were no sharp edges to catch on the rubber cork and then used a little spit and it went right in with only a little brute force.