First, do you have University installed?
That cheat was included with the University EP.
If you do have University installed, type it exactly like this into the cheat box:
boolprop allow45DegreeAngleOfRotation true
Rotate the object using the comma or period keys on your keyboard.
If you don't have University installed, you can try using this item (but don't know if it remains compatible with newer/newest EPs):
http://www.modthesims2.com/showthread.php?t=77257Should also add that certain items (some - but not all - drapes, mirrors, doors and windows, for example) just don't include a diagonal option, period. Certain other items might also require having the moveObjects on cheat in use simultaneously with the 45 degree angle cheat.