Ok to find the option go to Settings, then Security, scroll down to Unknown Sources and check the box next to it, as far as Covers/Cases goes their is an issue with the ones that have the small magnetic clasps, however i read up on this and its apparently a good indicator that their is a fault with the tablet as it shows that the shielding inside is faulty, however this has not been openly discussed, nor has it been proven that it has a link, officially , yet, but i went for a screen protector and a rubber gel protective cover, the cases i dont like, with the cover its just so much more accessible and doesn't burst at the seems after a few months, so your choice really