I managed to figure it out, so I thought I'd post what i did here. Please note, I am a newbie when it comes to this stuff so I might get something wrong.
The process is quite simple and assumes you have optware already installed
In a telnet session or the TomatoUSB Web UI > Tools > System Commands UI type
ipkg install nzbget
Now copy
/opt/share/nzbget/nzbget.conf
into
/opt/etc/
Now edit the conf file. The most important settings to look for are:
MainDir - This is where downloads go. Chances are you won't want this on your opt drive, so you'll have to change it to something like - /tmp/mnt/<partitionname>/dir
ControlPort - This will be the port that you will access Nzbget on. You shouldn't need to change it, but you'll want to make a note of it.
ControlUsername - The username to log on to the web ui with
ControlPassword - The password to log on to the web ui with
Now that that's done, all you need to do is start up the service. The command is
nzbget -D -c /opt/etc/nzbget.conf
You can put this in Administration > Scripts > Init to start it with the router.
You will probably want to also run the following commands
ipkg install unrar
ipkg install p7zip