you hijacked it so its your baby now 
… try reading thru this link it may help New XXL Z axis belt slips (tension bolt needed?)
also this S03: Assembly Guide (XXL)
and if that dosent help just put belt tension in the search and tons of stuff was coming up.I got mine going but really dont know how to describe the amount of tension on the belt,its kinda not to firm but not loose either,i havnt had it skip or anything as of yet.Im sure others with chime in to advise if needed,just post again with your results