When you connect a computer directly to the 300G+, and select DHCP on the computer, it receives its IP address from the 300G+. Initially, this address is in the 192.168.0.x range, which works fine to access the modem's HTML-based setup pages at 192.168.0.1. Later, when the ADSL connection is up, the modem will give your public IP address to your computer via DHCP.
When you connect an Airport to the modem, and use DHCP on the Airport WAN port, the Airport starts using some self assigned IP address from a different subnet and hence can't access the modem's HTML pages. You can, however, set the Airport's WAN IP-address manually to something in the right subnet and set up the modem. Once the modem connects to ADSL, you can switch the Airport back to using DHCP on its WAN port and it picks up the public address.
This is independent from the DHCP on the Airport's LAN side.
The problem may be on the modem side, but it does work fine with Macs for all IP-addresses and it does work fine with the Airport when it sends out an IP-address outside the 192.168.0.x range.
Note that you have to set up the modem from the (only) computer that will use it, in this case from the Airport. The 300G+ acts as a pure modem for one computer only, even if there are other computers on its ethernet.
This is nice if your ISP makes you sign an agreement that allows you to use only one computer at a time: you can always put away the Airport and connect and set up the modem from your computer, when there's trouble with the internet connection. The trouble wouldn't be the router's fault, but the ISP might try to blame it.
You can find other people's experiences at apple.com > support > discussions > network > airport > airport extreme > D-Link 300G DSL & AE - The Solution! (or search for "dlink 300g airport" in the discussions at apple/support)
You normally have to go through this setup only once, and the 300G+ is a cheap solution, so it may be worth the extra trouble.
