I had this error for a very long time when trying to use extensions in Inkscape 0.46 for Mac OS X:
“The fantastic lxml wrapper for libxml2 is required by inkex.py and therefore this extension”
After many hours of trying to find a solution I got it to work. This is what I did:
sudo easy_install lxml
mv libxml2.2.dylib libxml2.2.dylib.old
ln -s /usr/lib/libxml2.dylib
After that, all extensions (those found under the effects menu) started to work.
The problem seems to be fixed on the latest development version.
Fixed typo. Thanks to Bill Gathen for your comment.
Finally a version of Inkscape that works as expected out of the box. Tested on Mountain Lion. Link posted at the Inkscape development email list. Bonus: partial retina display support and no X11 required!
An up-to-date solution by Callum Vincent: