What you do is put in the boolprop cheat. Then when it's on click on the sim you want to be pregnant. There'll be an option saying spawn. Click that then tombstone of life and death and a gravestone appears. Click the stone and a load of options appear, one of which is 'make me pregnant with' which will give you MPREG.

And if that's clear as mud read here (http://www.livejournal.com/community/thesims2/324757.html) for someone who explains better than me *g*

Love the icon btw!