The whole works should be connected, buffered by the black rubber 'sleeve' that holds it in... If you push the cup down, the inside of the threaded end (the flat part with the hole in it) should pop out the bottom...
Or are talking about leaking from the metal tube at the base? That's another bunny rabbit altogether... I thought those were pressure fitted onto the base, they shouldn't be able to come apart (or leak) without a lot of force. You might just have a misshapen tube... I assume you're only having this problem with one carto?