In general, the procedure is: download the file dot2tex-2.8.7.tar.bz2. Don't extract it, just put it in the upstream directory of the Sage installation. Then run sage -i dot2tex. For this particular package, there is actually a problem: the tar file is outdated. You need to download the current one at http://boxen.math.washington.edu/home/vbraun/upstream/dot2tex-20120520.tar.bz2. I've reported the problem, also.