it should unlock automatically in the game if you have linked your EA account on a console or I assume just enter your email on the PC
you have to actually play the facebook app to unlock the items .. took around 10 minutes for the first one.
honestly all the unlockable items suck and by the end of the game I sold them all.
anyway,
21 hours for my first playthrough. ended up with 40/50 achievements (playing on xbox) .. so many missable ones! played as a rogue. they're massively overpowered in my opinion, I had an active buff that gave 15%+ attack speed or something, a 21% attack speed rune in my dagger, and a mage giving me an extra 60% attack speed for 20 seconds. I had some of the best gear in the game and by the end it was just a matter of spamming A and watching things die.
kind of frustrated that I missed so many achievements .. still pondering a second playthrough to mop them up. I don't know if I want to sit through it again, even though I'd be playing it a totally different way..
EDIT: This is one of those games where I found myself stopping to say "hey look this cave again. haven't I already been here .. twice?"