You don't just have one IP address within your home network. You have an IP for your home computer, your home router/modem, and the IP address that is assigned to you by your ISP that you can connect to from elsewhere in the internet. For example, my computer that I am currently on is set at 192.168.0.11. My router/modem is 192.168.0.1. But, my external address (this isn't actually mine, but youtubes) looks something like this: 220.224.223.109.
The range 192.168.xxx.xxx is reserved for private networks, ie your home network. There are also other IP address ranges as well reserved for private networks, but I highly doubt you'd have one of them. So, the address it is asking you to put in is either the IP address of your computer, or the IP address of your router/modem. Easy to find them, just open up command prompt (start, in the search box type 'cmd'). Black window comes up, type 'ipconfig'. Look for something like 'Ethernet adaptor Local Area Connection', and you will see two IP's of use, IPv4 address (your computer) and Default Gateway (your modem/router). I haven't looked at the portforward page, but it is probably asking for one of these.