The original replacement iPad 2 and iPad 3 digitisers with the original flex cables are designed in a way that the flex cable bends around the inside of the back housing. Take a good look as you are removing the original screen and you will see how it arcs.
When you fit the new digitiser the start of the flex cable has a firm part which is there to poke in between the LCD and back housing. The cheaper replacement digitisers lack this 'firm' part of the flex cable and are really hard to get the flex cable to shape as necessary for the glass to lay flat.
The original adhesive to fit the glass comes black in colour. You should never need any more adhesive than the black double sided adhesive.