You must download the Premium version of the app using the same iTunes account that you originally purchased it with. If you purchased the app (or the in-app purchase) using another account, we cannot connect the two purchases for you.
To look up your purchases for any given iTunes account, see this link:
If you are using the correct account and are trying to restore your Premium in-app purchase, just tap "Restore Purchases" in Settings. If for some reason restoring doesn't work (this can sometimes happen due to iTunes issues), you can tap to purchase the Premium in-app purchase again. iTunes will inform you that you've already paid for the upgrade, and you will NOT be charged again.
Note that if you downloaded the full-price app on the App Store, you should download that same app again in order to keep your Premium features. Similarly, if you downloaded the Lite version of the app and then paid for the Premium in-app purchase, you should download the Lite version again and then tap "Restore Purchases" in Settings.
The full paid version of the app and the Premium in-app upgrade of the Lite app are two separate purchases and we cannot connect the two for you.