Use an ascii text editor to check out the file. The header is the key (first line). Make sure it's proper for the rev of PADS Layout you're using. The easiest way to get the correct header is to export a netlist of any pcb you have. File> Export>, then select .asc in the File of Type, give it a name and direct to a folder that's easy to find. After you export the file, use your test editor (notepad works great) to open the ascii export. Find the header and copy it. I use 9.2 for example, and my header looks like this:
!PADS-POWERPCB-V9.2-BASIC! DESIGN DATABASE ASCII FILE 1.0
Yours will vary. Paste it into the OrCAD netlist (removing the incorrect one), andsave. Now you should be able to import just fine.