The plugin won't launch. Why?
The first time OnSync launches the screen sharing plugin, you will be presented with a dialog that asks you to accept the execution of the plugin. The best thing to do is to put a check inside the "remember my choice" checkbox and to press the button to accept the application to launch. On a Windows XP computer running FireFox, this dialog looks like this:
If you put a check inside the checkbox and then press "cancel", FireFox will remember your choice to disallow the launch of the plugin. The problem with this is that you will not be shown the dialog again, and you will not be able to run the plugin. To fix this, you must type "about:config" into the FireFox address bar, and enter "launchappshare" as the filter term:
Then select the launchappshare item and right-click it. From the dropdown menu, select the "Reset" option. This will reset the "remember my choice" feature in the dialog, and when you try to launch the application sharing plugin, you will now see the original dialog requesting your permission to run the plugin. This time, please check the "remember my choice" option and accept the execution of the plugin.
NOTE: this article needs a revision. On my Vista/FF3.0 the about:config does not show launchappshare. Instead, I need to look in Tools>Options>Applications (see here), and in the session, I see the following dialog. If the exe does not exist for some reason, it will not allow me to press OK.