Liskell is available from this darcs repository:

• http://darcs.liskell.org/ghc-6.6/ghc

Liskell is a regular GHC branch and builds identical to a regular GHC branch:

darcs get --partial http://darcs.liskell.org/ghc-6.6/ghc
cd ghc
chmod +x darcs-all
./darcs-all get
autoreconf
./configure
make

To get the Liskell testsuite go into your ghc directory:

darcs get --partial http://darcs.liskell.org/ghc-6.6/testsuite 
cd testsuite/tests/liskell
make stage=2 fast

The GHC Develop Wiki is a very valuable resource for GHC Hacking. Useful direct links are:

• GHC Trac - Getting the GHC Source — You just need to replace http://darcs.haskell.org/ghc with http://darcs.liskell.org/ghc-6.6/ghc in these instructions.
• Quickstart - Build instructions