No phone line is connected. The dish & modem do 2-way communication. I have Airport Extreme about a metre (3') from my Mac. However, I had the same problem when I bypassed the Airport and connected directly to the modem by ethernet cable.
OK, good deal.
Here's something to try on the Mac:
1. Open System Preferences and go to Network
2. Choose the network adapter you're using to connect (I'm assuming WiFi if you're using the AirPort Extreme).
3. Click the Advanced button
4. Click the DNS tab
5. Under DNS servers, click the Add (+) button
6. Add 8.8.8.8 and 8.8.4.4 (these are Google's free public DNS servers - if you're wary of Google, we can try a different DNS service, but let's just use these for now - we can undo it later if it doesn't work)
7. Click OK
8. Click Apply and close System Preferences
Now, try Help Centre once again (you should not need to reboot). If it works, the problem is likely with your ISP.
If you want to revert, go through the same process, but use the minus (-) button to remove the addresses you entered. System Preferences should revert to the DHCP-supplied DNS server name (likely 192.168.1.1, which is your AirPort Extreme acting as a relay to your ISP's DNS servers).
If you don't want to use Google's DNS, you can also try
OpenDNS, but this requires signing up with an account. Once you have an account, you will enter the DNS server addresses in the same manner as described above.