how do you unlock your iPad if you forget the passcode

Try and force your iPad into Recovery Mode:

1. Turn off iPad

2. Connect USB cable to computer; leave the other end alone

3. Press and hold the Home button down and connect the docking end of cable to iPad

4. Continue holding the Home button until you see the "Connect To iTune" screen

5. Release the Home button

6. Open iTune

7. You should see "iTunes has detected an iPad in recovery mode"

8. Use iTune to restore iPad

Note: You need to be patient and repeat the above many times to recover your iPad

You can't.

Follow the instructions in the support article you arrived here from (linked at the top of this thread).