If you have (or can install) the Haskell Platform this is the next best option, as this will resolve build dependencies automatically. To download, compile and install Darcs and its dependencies via cabal-install, the build tool provided by the Haskell Platform, simply run
cabal update cabal install darcs
If you cannot install the Haskell Platform or cabal-install, you need to run
ghc --make Setup ./Setup configure ./Setup build ./Setup install
This will require the following build dependencies:
Additional build dependencies are declared in the darcs.cabal file, and ./Setup configure will tell you if any required build dependencies aren't found.