In order to access the Menu button, you need to unlock the tablet. If you don't remember the pattern/PIN number and if you have Internet access, I suggest you install Screen Lock Bypass Pro from Google Play Store. It will bypass the lock screen and will reset/remove the screen lock password, pattern or PIN number. If this doesn't help, I suggest you follow the instructions from the tablet's guide and perform a factory reset. Basically, you need to turn the device off, to press Volume Up + On/Off buttons and to select Wipe Data/Factory reset from the Recovery menu.