Port-forwarding problem- LAN Clienting

[Dark Crusade]

I successfully portwarded about 2 weeks ago, and i was able to get my game online and have other people join it. But now i am having an unexpected problem.

No one has been able to join my game, i have given them the correct IP address etc. I went into my modem settings and it would not let me open the portwarding page. When i try to do so, it gives me a message saying "You must enter an IP address on the LAN clients page for the selected LAN group to enter this setup"

So i went into the LAN clients page, and it asked me to fill out:

Enter IP address:
Hostname:
Mac Address:

What would i put in these boxes? Or where would i find out what to put in these boxes? I tried putting my external IP address in and making up a random hostname but that didnt work. Please help..