(Tested with Ubuntu 12.10 32 and 64 bit)
sudo apt-add-repository ppa:vala-team/ppa
sudo apt-get update
sudo apt-get install vala-0.20 valac-0.20 git libgtk-3-dev libsoup2.4-dev gir1.2-json-1.0 libjson-glib-dev gir1.2-gee-1.0 libgee-dev
git clone git://github.com/JumpLink/magento-gtk.git
git clone git://github.com/JumpLink/vala-magento.git magento-gtk/src/dependencies/magento
git clone git://github.com/JumpLink/vala-heritage.git magento-gtk/src/dependencies/heritage
cd magento-gtk
make
./magento.o
First modify src/config/org.jumplink.magento.gschema.xml.example and save it as org.jumplink.magento.gschema.xml.
sudo cp src/config/org.jumplink.magento.gschema.xml /usr/share/glib-2.0/schemas/
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/