@darkdrea If the Pet Shop venue has a microwave where the register should be, that likely means the register is not installed at all, so your game replaced it with another object that it thinks is similar. (The microwave seems to be the consensus replacement for the register, but why is anyone's guess.) When it's installed, you can find it in build (not buy) mode, under community objects/misc.
If you can't find the register, you may have a bad install or even a corrupt TS3 folder. You can test this by pulling your existing game folder out of Documents and onto your desktop and then launching the game to spawn a new one. After you quit, you can download only Graham's pet shop into this new folder, and then start a new game so see if the register appears. If it does, you can transfer your saves and other content into the new folder from the old one. You will need to redownload and reinstall your store stuff, unless you want to go through all of your .ebc files to find out which one contains the store.