I just discovered that there is STILL a problem with PatternViewer.
When it is the "last" installation of M&R Tech products, it steals control over the .pat extension. In this mode, a double-click or right-click+Select option works and the files will open in PatternViewer. However, if another M&R tech product has control over the .pat extension (such as PCStitch Pro 9), a right-click+Select option will result in the error reported.
If you OPEN Pattern Viewer and the from there open the file, it will open fine. PatternViewer just does not play well with the other products. It wants all the resources to itself and being just a viewer, this is not right. It should automatically take a subservient position with respect to the PCStitch product line.