Go here: http://releases.mozilla.org/pub/mozi...ning/releases/
Then start digging through the directory structure. The native 64-bit .xpi files are in the
folder for each release.
Some of the releases only work with some versions of Thunderbird. I'm using the 64-bit version of Lightning 1.0b1 with Thunderbird 3.0.8 on Slackware64-13.1 -stable.
I have installed and enabled multi-lib, but that has nothing to do with the 64-bit software like Thunderbird and its plug-ins (I also use Enigmail).
If you're on -current, you may need a different version of Lightning to go with the newer version of Thunderbird there.
Good luck, and let us know how it turns out.