I just made my first patch to the darcs source, a minor fix correcting an incorrect help message that has always bugged me.
Stuff I have learned:
There are two different build systems with darcs: cabal and make.
Cabal install does the usual thing
Make is what you want if you want to load darcs into ghci. You need to run autoconf first, which means you need autoconf installed — apt-get install autoconf on debuntu. Then just autoconf; configure; make ghci.
Beter to hack on darcs during daytime pacific time, when there are people on the #darcs channel to answer questions. It’s a lot of code to grok!