You can also plumb the shop-vac into the waste line. Duct tape is handy for this. Then plug the skimmer with a gizmo and put the multiport valve in some intermediate position to basically connect all the ports. With the skimmer plugged the air will be forced to the return line. I think this should work for you. Compressors are usually low flow, high pressure units. You want high flow, low pressure to blow the lines. Thus a shop-vac. You can easily check if the blower can develope enough pressure by simply sticking the blow hose in the pool to the depth of the return jets. If you still get air flow and bubbles then the blower can blow the lines clear. I use the term shop-vac as a generic. There is a real company called that. Some units don't really develope much pressure on the blow port. They vent a lot to the outside and this reduces the available pressure at the hose.
Al