Normally you would login to the router with its IP address. This is normally something like 192.168.0.1 (you would have to check your specific routers documentation)
There should be an option within your router to setup port forwarding.
iWatchMobile and ActivePhone use port 8777, so this is the port to forward.
You will need to know both your internal and external IP.
If you have a dynamic IP (changes frequently) I would look at using a free service like DynDNS.com.
This will allow you use a domain name in place of having to remember updated IP addresses. Eg: Always type the same address in this format
http://myahpcomputer.com:8777/iwatch instead of a different IP address such as
http://0.0.0.0:8777/iwatch. If you know your external WAN (internet) IP, or you have a static IP, chances are you won't need to worry about this.
I'll attach an image that shows a basic idea of what you need to setup for port forwarding.
In the IP box in the port forwarding settings you would type the computers internal LAN IP that is running AHP.
This will direct an outside connection on port 8777 to that specific internal computer.
From your BlackBerry browser you would type
http://yourexternalipaddress:8777/iwatch or click the email link you generate in AHP under plug-ins/iWatchMobile/Invite a cell phone.
Hopefully you can make some sense of this.
If you need any more info, just let us know.
As far as I know, x10 does not currently support 3G connection info in their setup instructions for this where it is more involved and has such a wide number of variables because of so many different brands of routers.
The picture I attached is an emulation page of a D-Link router. If you use a different brand router it may be slightly different to setup.
If you are unable to get it working, letting us know the specific make/model of your router may help.