PD60 RCT-10-L Low Shank Screw On Side Cutter Serger Cutting Attachment

I recieved a side cutter 11 as a gift but the box doesn't contain any instructions. The box says to attach it the machine by replacing the presser foot. I've done that but I cannot get this attachment to work. I have a White model #999. Will this foot fit my sewing machine? Am I just putting it on wrong? Thank you for any help you can give me.
You have to put the plastic fork over the needle bar clamp while you are atttaching the Side Cutter the presser bar with the presser bar screw. As the machine is running, the forked lever goes up and down with the needle and activates the cutter with every stitch. Then you have to select straight stitch for french seams, zig zag or overlock stitch for single layers or double overlock stitch for seaming two layers. For seaming two layers start a 1" cut 3/8" in from the fabric edge with scissors so the seam allowance overcasted is 2/8" or 1/4". You have to start the cut so the fabric splits at the knife but reaches the needle on the left to start cutting and sewing. Its not like a serger where you can just stick the fabric under the foot and it will start cutting and sewing. If overcasting one layer of fabric with the overlock stitch you can start without cutting with scissors, since you can lay the raw edge of fabric up against the knife and under the needle at the same time. The Side Cutter should fit all low shand screw-on foot presser bar machines. -John Douthat

I've sewn about 12 baby fleece wipes for my baby w/ no problem and then the side cutter wouldn't work. Why does the side cutter stop working on the fleece?
You will need to press the seam allowances by ironing out the fleece.
-Annette