The only way to access the iPad is by remembering the passcode. If you can't remember it then you will need to reset the iPad, but if you have a backup you should then be able to restore that back onto the iPad. You may need to put the iPad into recovery mode in order to be able to reset it : http://support.apple.com/kb/ht1808
If you do it via a computer that it's not synced to then (from here) :
If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.