go to MOHAA main menu > Multiplayer > Join game > Internet game. Under the 'Connect to IP Address' button enter the IP number (the number you have) and hit the 'Connect to IP Address' button. Then the map will load if you connect to the server successfully.
Welcome to the forums Homer. When I started I did it the way that Silver suggests. No questions are stupid - the first time I wanted to go online it took me a while just to realize that I had to go through the door on the right side of the main menu to get to the screen that allows you to enter the IP address, etc. - doh!