If Outlook is the one in your start menu, it is already assigned as the default client unless you manually changed it in the start menu properties. From there it's possible to show a non-default mail client on the start menu. Since it appears that it's already your default, it won't be available in the Set Program Access and Defaults program except as the "use current option" option, same as my machine.
Are there any updates you can apply for Groupwise? Since I can't test in your exact environment I can't suggest much else...
