About device> click many times on the "build number" thing and you will be able to use Developer Option!
It's a very old thread I know! But I answer this if some newbie sees this thread and has the same problem!
Re: I regret rooting my tablet, Help!
I've had the same problem! I unroofed(like you did) and factor reset :\ It didn't work! I just installed Lollipop using my computer(I'm a root-noob too)