Warning: this requires lot of magic
Get the latest source from git: git clone git://git.bitcheese.net/hellnet
Requirements are: GHC 6.12 (didn't check with other compilers), Cabal (you will have to install them manually, as described here) HTTP, Lucu, Crypto (these are Cabal packages, you can install them with cabal install name)
Once you have all those installed, Hellnet can be built with
runghc Setup.hs configure
runghc Setup.hs build
runghc Setup.hs install
On first stage, Cabal may complain about missing dependencies, you'll have to install them.
Alternatively, if you don't want to mess with Cabal, you can just run 'make', it will compile all the executables in current folder. They are static and can be safely moved.
Hellnet is available on Hackage. To install it, you'll need working Cabal setup (see above). Then
$ cabal install hellnet
And pray, since not all packages on hackage are ready for GHC 6.12 and you may have to patch them.
Last resort: download jumpstart package, extract & launch.
Now, to the next step, use.