If you try to add a server, but get a message that your server is not on (Offline), and you are sure that the server is working, then most likely you do not enter the Query port.

In such cases, you need to find out exactly what value of the "net_queryPort" setting is installed on your server. This port must also be specified when adding a server.

For example, your IP port is 27015 and you found out that "net_queryPort" is 27014.
In the form of adding a server, you specify the following:
Server Type (Game) *: Call Of Duty: Modern Warfare 3
IP *:
Port *: 27015
GameSpy (Query) port: 27014
Your E-Mail *: ...