After making the compact.xml, you have to convert to a lcdd file ( for SLIC) and a heprep file (later, for org.lcsim and wired). both can be done using GeomConverter.
The next step is to make a little macro for SLIC to use the particle gun and generate a few events. This is sample:
/lcio/PDGFlag true /generator/select gun /gun/position 0 0 0 /gun/direction 0. 0. 1 /gun/energy 10 GeV /gun/particle neutron /lcio/filename slic_clichcalstack_neutron_10GeV_Wscint.slcio /lcdd/url /home/marcel/demo/stack/clichcalstack.lcdd /run/beamOn 125
The particle Gun is set up as
/generator/select gun /gun/position 0 0 0 /gun/direction 0. 0. 1 /gun/energy 10 GeV /gun/particle neutron