unlock ipad in voiceover mode

With VoiceOver 'on' you need to tap to select and then double-tap to activate - so to use the slider tap it (and it should then get a box around it) and then double-tap it to slide it, and then do similar for your passcode to unlock it. To get to Settings > General > Accessibility you may need to use 3-fingered scrolling on the screen.

You can also turn it off by connecting to your computer's iTunes : http://support.apple.com/kb/HT4064