I have installed this solution (running GroupWise 6.5.3) and it works for me; I can run it by clicking the item Add/edit message note on the message list context menu.
It sounds like you have both secure and open source versions installed. Formativ Runtime will not execute an open source solution, nor integrate it with the GroupWise toolbars/menus. The presence of an open source solution, along with its closed source counterpart, may be preventing Formativ from handling the closed source solution normally.
I would suggest that you remove the file add or edit message note.vbf, restart GroupWise and try again.
You can determine which solutions are available to Formativ (given a standard installation, there are several possible locations) by looking at your Formativ Configuration. You can obtain the configuration by selecting Help | About Formativ… from the GroupWise main menu. When the About Formativ dialog appears, go to the Configuration tab. Scroll to the end of the text to the section Available Applets where you will see their complete file names listed.