Try it as written at the link Auronette gave, including the caps:
boolprop allow45DegreeAngleOfRotation true
Should work so long as you have at least one EP installed. If only playing base game, you cannot use this cheat.
Some computer/game systems will accept cheats without caps -- all computer/game systems will accept cheats with the caps.
You can permanently include this cheat in the game by putting this cheat into your userstartup.cheat file. See here:
http://www.sims2wiki.info/wiki.php?title=Game_Help:Userstartup.cheat