Hi, I am missing something i am using for a long time, but i do not know if it's a Gimp issue or a ufraw plugin issue :-D
I use Gimp with UFraw for Canon .cr2 files, which is just georgeous, since i have a lens correction ta included …
Now i have a new laptop, running ubuntu 12.04 64bit, gimp 2.6.12 and ufraw plugin 0.18 1.1 installed right out of the ubuntu sources via synaptic - but there is no tab with lens correction in the plugin.
You're probably missing the lensfun libraries. Not a Ubuntan myself but they should available be in your repos.