Hello. So you made a copy of the WhatsApp/Database folder correct? Good.
1. Go into Settings > Apps > Whatsapp.
2. Then click "Clear Data" and "Clear Cache"
3. Then open up WhatsApp Again
4. Close out and Uninstall WhatsApp
5. Download WhatsApp from the Market again, DO NOT OPEN IT.
6. Copy the backed up Database folder into the original position.
7. Open WhatsApp, Put in your phone Number and Push Notification Nickname, hit next
8. You should then be prompted to Restore a Backup, Hit Yes and continue.
9. Messages should be restored.
I Hope this helped. I Only know it because when i change ROM's i loose the conversations with my lady and its a right pain. So i learn't how to back it up, i now keep multiple backups.
Hope this Helps, Hit thanks if it did!