At the moment, you have to build Atomish from source. Also, only the bootstrapping pre-interpreter exists, which interprets a pre-Atomish language.
You will need a Java 7 sdk, and a recent Scala (I use 2.10.1 for development). Both should be available from your package manager, or as direct downloads from Oracle (probably Technetwork downloads) and http://www.scala-lang.org/.
Then from the project root:
cd bootstrap
./mk.sh
Once you have built it, the preinterpreter can be invoked through a shellscript found at bootstrap/bin/preatomish.
The script bootstrap/lazyinstall.sh, if invoked from within bootstrap (and with the necessary permissions, so probably as cd bootstrap; sudo ./lazyinstall.sh), will make a symlink from /usr/bin/preatomish to bootstrap/bin/preatomish, thus putting preatomish on $PATH in the laziest way possible.