You download, install and run it. Then you click on the cog icon and you select the Join existing network option. You can join one of many OJP hamachi channels readily available from there. The single most used one would be:
OpenJediProjex 1 / ojp / OJP (I forgot the password, it's one of these. Try them all with and without spaces and you should figure it out soon enough).
I can't ever join Hamachi I think they actively reject my IP. Everytime I tried it just said failed to connect and I was rejected by the servers. So I've not been able to help test or anything like that or even know what server you guys use.