If you are trying to purchase for the first time today, take a look at this article which provides detailed troubleshooting information. If your purchase was more than a day ago, take a look at the suggestions in this article.
First, double-check to make sure you are looking at the correct license. There are separate licenses on the Market for the smartphone and the tablet edition.
If Google Play is asking you to pay again, this indicates that the store is linked to a Google account that is different from your original purchase. To check, open Google Play, and press Menu → Accounts and make sure the correct Google account is selected.
If the Google account is not listed, then you will need to go to the “Accounts” area of your device’s settings and establish a link to this account. Once you have done this, you can go back to Google Play, select the correct account, and try downloading again.