it's fixed in this bug Bug 34342 – Use Zimbra Desktop as default mail application does not work please try again in the coming release
ctrl + click, can't be much easier, thanks for the quick reply!
When I go to "preferences-global-system defaults" the box is not active. Is there some problem that I should know about? I am using Vista, and have Outlook software which was my original default. If I try to change the default at the control panel level, I am also not allowed the option of choosing Zimbra.