To get started jamming you'll want to download a client app.
Install the Client Software
To get started you will need to install an audio client
app on your computer. There are several choices.
JamTaba - available on Windows, Mac Osx and Linux. It works as
both an application (standalone), a VST (windows) or an AU (Mac Osx)
Zenjam is a free standalone application which is easy to install and use.
It runs on Windows, OSX and Linux.
Reaper - you can connect to NInbot Servers from the the ReNinjam plugin. Reaper also allows for other VSTs to be used with it. Reaper support OSX/Windows.