Re: New build locked up.
You have to make sure the rear sprocket is on the wheel almost perfectly, no side to side or up and down wobble at all. 99.99% of all chain problems can be traced back to the sprocket not being mounted true. Also if you haven't bent the tensioner bracket to allow the tensioner to run in the same line as the chain that will cause the chain to come off as well. Your tensioner has to be bent to take into account the angle of the chainstay.
Can you get us any pictures of your setup? They really are worth a 1000 words.