You normally enter the special codes into the shop - they are not really "cheats" - just a few items that you can get for free (first time around).
Lots of ppl have problems getting the shop keeper suprised and getting all the codes into the tama - I have had problems with the shop codes on the V2, V3 and V4
All I can suggest is that you keep trying. That's what I did and eventually I got the tama to accept the code and it gave me the free item
Perhaps you aren't pushing the buttons rapidly enough, or you're pushing them too fast. Make sure you distinctly press A three times. It usually takes a couple tries; I sometimes get frustrated with my v3! By the way, if you don't know what debugging is, then that's definately not the problem!
Normally,When Your Shop Items Are Full,You Can't Enter The Codes.Try To Delete Some Items And Food Items. If It Is The First Time You Are Getting A Item But It Cannot Be Entered,You Might Want To Try Again.