I was able to install and run the 2.5.1 Intel version of the Workplace Forms/Pure Edge Viewer on my MBPro, (Intel-based), with OS X 10.6.1 Snow Leopard.

1) You must right click the .pkg file and Show Package Contents.
2) Find and right-click the Contents, Resources, InstallationCheck file and Open With your TextEdit app.
3) Search for the text "10.4" and replace with "10" then save and exit the text editor.
4) You should be able to close up and double-click the modified .pkg file to install the viewer and finish with a restart.

It "sort of" works in that not all buttons function and I cannot "save as" or print... I'm still working on that, but at least you can view .xfdl files.