Mac error message: There is no default application specified to install the document...

This rare message comes up when the Mac Launch Services database does not work properly or is corrupted. It is a problem with your Mac, and not specific to any GraphPad program.

The problem is easy to fix:

If the message is still up, click the button; Choose application. Then choose installer.app which is located in:

[OSX 10.5, Leopard]  /System/Library/CoreServices/ 

[OSX 10.4, Tiger]      /Application/Utilities

Alternatively, control-click on the installer icon, choose Open With, and choose Other...  Then, in the Open dialog, choose installer.app

This problem has been discussed on apple.com here and here.