Hye these are the steps you should use to use cheats on sims 3:

press Ctrl Alt Windows (should be next to ALT button) and C all at once. Then you may enter any cheat you want]
After enabling the test cheats, you can use any of these cheats:

Cheat Code - Result

help - Display most cheats
Kaching - 1,000 more Simoleons
Motherlode - 50,000 more Simoleons
shazaam - +2,500 lifetime happiness points to active Sim
freerealestate - Ignore the cost when buying a lot
modify traits - Set or clear traits
add to household - Add active Sim to current household
set age [number] - Set age of the Sim
edit in cas - Take the Sim back to Create-a-Sim
set career [career/level] - Give selected Sim a career
force opportunity - Click career building you work at to force an opportunity
force event - Click career building you work at to force an event
force all events - Click career building to display all events for the career consecutively
force service sim [name] - Force a specific service Sim to appear
force visitor - Forces a neighbor to show up
make me know everyone - Selected Sim knows every other Sim
make friends for me - Make several random friends for selected Sim
make happy - Sets all mood/moodlets for everyone in the house to perfect; removes negative moodlets
make motives [static or dynamic] - Set motives static or dynamic for entire household
unlockOutfits [on or off] - View career outfits and service uniforms [Note 1]
moveObjects [on or off] - No limitations for placing/moving objects
resetSim [first name] [last name] - Return Sims to safe and neutral state at home
constrainFloorElevation [true or false] - Force terrain adjustments regardless of objects, Sims, and other structures
disableSnappingToSlotsOnAlt [on or off] - Objects will not snap to slots while holding [Alt]
fadeObjects [on or off] - Objects fade when camera gets close to them
snapObjectsToAngle [true or false] - Toggle 45 degree angle object snap
snapObjectsToGrid [true or false] - Toggle snap to grid for placed objects
hideHeadlineEffects [on or off] - Show talk/thought balloons above Sim's head
jokePlease - Print random joke to console
enablellamas [on or off] - Llamas enabled message
slowMotionViz: [0-8] - Slow motion visuals; 0 is normal, 8 is slowest
testingCheatsenabled [true or false] - Enable testing cheats [Note 2]
fps [on or off] - Toggle framerate display at top right
fullscreen [on or off] - Full screen
quit - Quit the game

Did you get a update for the game?