It does not look as if there is a ZS version of Snort yet for beta 13. I have not compiled any addons so I cannot answer howto: but there is a Snort that works on beta12, depending how much was done on your ZS beta13 setup you may want to use beta 12 till one is written or wait till someone can compiled for you or tell you how.
patches are located here http://zeroshell.net/eng/patch-details/