try www.brother.com www.manualsonline.com
and see if this manual is available. You may have to pay for it however, most manufacturers view a manual as a spare part to be charged for.
Most overlockers are very similar but if you have never used one at all, then well worth going to a demonstration or class at local store as the techniques to use one are quite different to sewing machine sewing.
Threading path is the critical thing and once mastered, using them is pretty easy.