There is a newer applet on Cool Solutions that I think might work better for you. CSV Address Book Importer takes a CSV file and imports to a GroupWise personal address book. This is an installer for the applet.
You can get Excel to create a CSV file from your Excel address book. Choosing from the main menu File | Save As…, then set the ‘save as’ type to CSV (Comma delimited) (*.csv). Then follow the instructions included with this new applet.
If the problem you are getting continues, please send us your Excel address book, to firstname.lastname@example.org