To 'install' the stamp collections, please do the following:

1) Open another Windows/File Explorer window and copy/paste the following path into the address bar and hit 'Enter':
%appdata%\Tracker Software\PDFXEditor\3.0\Stamps

2) Move or copy/paste the 'Checks and Marks.pdf' file to the other Explorer window (%appdata%\Tracker Software\PDFXEditor\3.0\Stamps) that you just opened.

3) The stamps should now be available for use in the Editor.