Although Darwin comes with the dpkg utilities, it has no dpkg database, so it is very difficult to use with Darwin. The easiest way to install .deb's under darwin is to do it by hand. Remember that a .deb is just a bunch of .tar.gz files ar'd together. So to extract the data from a .deb file you can do the following:

ar x file.deb

This will give you several files, but the important one is data.tar.gz. This is the actual data files from the .deb. Then you can cd / and extract the data.tar.gz file. You should use gnutar to extract the file, because of minor behavioral differences between the BSD tar and GNU tar.