why after syncing is my version still at 4.3.5?

Syncing is not the same as updating. Did you update the iPad via iTunes on your computer? You have to select Update within the Summary Tab of iTunes while the iPad is connected to your computer, with iTunes running and your iPad selected on the left side under devices.

Did you do that?

Syncing alone won't update the software. You have to select the Update button in iTunes and go through the update process.

Connect to iTune and click "Check for Update"

http://i1224.photobucket.com/albums/ee374/Diavonex/154da938.jpg

Just syncing won't update the version. Have you got the current version of iTunes on your computer ? If you have then you should be prompted as to whether to update to iOS 5.1 or not. If you aren't prompted then with your iPad connected to your computer and selected on the left-hand side of its iTunes, on the Summary tab on the right-hand side click the Check For Updates button.

Before starting the update process, copy any purchases of the iPad and force a backup : http://support.apple.com/kb/HT4972