There is a huge difference between "PADS" and "PADS Pro". The latter is a stripped down version of Expedition / xDx Designer / xPCB Layout / Whatever funky term the Mentor/Siemens marketing department thinks up tomorrow.
Altium designer has an import wizard. Try using various options that contain the term "PADS", "DX Designer", "Expedition", "Mentor" and "Viewdraw" until you find the one that imports for you.
Warning: have aspirins or stronger painkiller nearby.