I have Adobe LiveCycle Designer 8.0 installed (came with Adobe Acrobat 8.0 Professional) on my PC. I also have Adobe LiveCycle WorkBench installed on my PC. When I tried to create/open a pdf/xdp file from WorkBench, I got "Incompatible LiveCycle Designer Version" dialog box poped up with the following information:
The active version of LiveCycle Designer is not compatible with this version of WorkBench.
Please lanuch a version of LiveCycle Designer 8.1 or later and make it active, and then try opening your form again.