This was not as simple as I first thought. The thing was that I had installed the iWorkTrial first and used it a couple of days to see if it was to my liking, and it was, so I bought the family pack. Now the trial version needs a serial number to become a full version and the family pack does not come with a serial number.

I first thought that if I installed the family pack iWork everything should med fine. This was far from the truth. Every time I started one of the programs (Pages, Numbers or  Keynote) it asked for a serial number (just as the trial version). I googled a bit and it seems that you need to remove the old iWorkTrial09 first. Just removing it from the Applications folder is not enough. Here is what I did:

1. Move the /Application/iWorkTrial09 to the Trash
2. Move the /Library/Application Support/iWork09 to Trash
3. Move com.apple.iWork.Installer.plist, com.apple.iWork.Keynote.plist, com.apple.iWork.Pages.plist from /Users/yourusername/Library/Preferences to Trash
4. Move all iWork, Pages, Keynote, Numbers pkg files from /Library/Recipies to Trash
5. **** IMPORTANT **** Empty the trash!!!!
6. Now install the full version from the family pack DVD

This worked for me. Hope it works for you too!