In order to build muttator, you also need to checkout vimperator from:
http://vimperator.mozdev.org.

And symlink the following files:

cd muttator/src
ln -s ../../vimperator/src/Makefile.common .
ln -s ../../vimperator/src/content .

If you want to check-in changes, don't forget to also commit the vimperator part
inside the local vimperator/ directory.