It looks like you are using the latest stable release 0.9. This is the latest release which works with TB2. To get Lightning for the TB3 beta's, get a nightly build. You can find a link to the nightly build for your platform at the bottom of the download page:

  http://www.mozilla.org/projects/calendar/lightning/download.html#nightly

Cheers,
Stefan.

> So far TB 3 doesn't work with the Lightning Calendar add-in so I am stuck with TB 2.X for that.

Eh? The reason I am using Thunderbird 3 beta is because that allows me to use Lightning Calendar 1.0pre.

You should give it a try.