I'm assuming I connect to Game Arena and don't have to make up my own server.
yeh that's what you've just bought off gamearena
I think the URL you want for your server info should be something like
http://secure.gamearena.com.au/account/serverrental/manage.php - then click manage. I just a bought 10 slot server as a test and that's the URL I get. In that page you want the value from 'Physical Address' - it wil show an ip address and a port number on the end, separated with a colon which is standard notation for ip:port
take the ip part and put it into "Hostname or IP" in vent
take the port part and put it into "Port number" in vent
stick the password in as well if you've put a password on the server via the above URL
default channel you can leave blank for now