If you like life on the bleeding edge you can get the very latest development snapshot via git:
git clone git://github.com/mythryl/mythryl.git
This will give you source code only. Since Mythryl is a self-compiling system you will also need a set of seed binaries to build the system. These may be obtained initially from the bin/ directory of the tarball download (preceding section).
If you do not have git installed you will have to install it. Under Debian, Ubuntu and related Linux flavors this will involve some command like:
sudo apt-get install git-core sudo aptitude install git-core sudo synaptic install git-core
There are a variety of git-related tools available. On Debian Linux and its derivatives like Ubuntu you can try searching for them using one or more of:
apt-cache search git apt-cache search --names-only git debtags search devel::rcs | grep -i git
Many thanks to the near-omniscient Drake Wilson for the above search hints!
For more information on git see the online user manual.