I'm not being rude and thinking you are stupid by saying this so please don't take offence!
WHen you go onto Google play and push the menu button to go to 'MY APPS' it then shows 'INSTALLED APPS'. Swipe to the left and it will come up with a list of all apps (ones purchased and are not installed should be there).
If you do that and it is not there try to find the receipt from google that they email you and make sure you are on the right google account. If you are, contact the developer/google play to complain forwarding the email receipt to them.