I have done a fresh install of Ubuntu 11.04, and then installed Thunderbird. I have tried to install two extensions, Lightning and then Provider for Google Calendar. Neither add-on will show up in the add-ons menu after the install and reboot of Thunderbird. I have installed Lightning both manually by downloading the .xpi file from the add-ons site and also trying the built-in utility for downloading add-ons in the Thunderbird menu.
Its strange that the ubuntu repositories does not have thunderbird-lightening installed in it.
Best Answer
The lightning extension is now available from the repositories for 11.04 (natty), 11.10 (oneiric) and 12.04 (precise)
By installing
xul-ext-lightningthunderbird will have a calendar available to add events etc.Note - you may be warned about installing the add-on when launching thunderbird up-to-three times. Each time, tick the box to install the add-on and click the Restart Thunderbird button.