Yes, you will need to put your static IP into your client to access your VPN server. No, you don't need to enter it anywhere in iVPN/easyVPN.
You should assign the Mac running iVPN a static IP in your 192.168.xxx.xxx range (but outside the range given out by DHCP). For example, if you are giving out 192.168.1.100-150 via DHCP, assign your Mac 192.168.1.50. In your Airport Extreme/router forward the following ports to your Mac (192.168.1.50):
Then turn iVPN on with the appropriate PPTP or L2TP choice depending on which ports you forwarded.
On your client, access it via the static IP given to you by your ISP. Your router will receive the request on the ports you forwarded and forward it to your Mac running iVPN and the VPN connection will be established.