I have version 3.1 running in both my production 0.4.2, and in today's 0.5-rc. The plugin works on 0.5-rc, but there is a bug.
When you click the button, you get the warning that you are leaving the compose window! (See attached) Obviously, if you cancel nothing happens. If you click OK, you do NOT leave the compose window, but instead the addressbook search opens.
Once past the false alarm warning dialog, it works as expected.