i would look at the copious notes you (hopefully) took during your entire brewing process and try to identify possible causes. cross off water quality unless you live in bangladesh or there was a massive water main break in your area recently. water quality throughout the united states is perfectly fine. it is possible but an extremely unlikely cause.
after reading this thread it appears no one pointed out an unusual thing you did: pitching yeast a second time 72 hours deep into fermentation. this is not normal. with a dry ale yeast at the temps you mention, you should have had no problem with the initial yeast fermentation at all. at all. temperature ESP. 70F for an ale could not be your problem. that rules out water quality and temp in my book.
my guess...you didn't use high enough quality ingredients or you (i think this is unlikely) got an infection in the wort that inhibited respiration and fermentation of the yeast. i personally think you should use higher quality ingredients next time and not a mr beer kit. i've never used mr beer but any HB company called Mr Beer doesn't exactly strike me as one that might be inclined to provide high quality ingredients in its product. i don't know though dude. some people like mr beer.
it doesn't matter though dude. just keep brewing and taking notes. especially note what problems you had and write down any questions you have, then post them in here to get answered. you won't have this problem next time if you are determined to succeed.
