Andrew, please don't post duplicate threads. I was just about to respond to your other thread... glad I saw this one first.
Firstly, this is not an ISPConfig issue. This is a basic networking issue.
That said, you need to create two records in your GoDaddy control panel: an "A record" and an "MX record" (the latter is used for mail).
This link includes you everything you need to know:
Point both the A record (yourdomain.com) and the MX record (mail.yourdomain.com) to your ISPConfig server's IP address, save the zone file, and you're done.