One of the following suggestions may help you:

Your server is currently offline.
GameTracker needs your server to be online before you can add it to our database. Please restart your server and make sure it's up before hitting ADD SERVER again.
You entered the wrong query port.
Please update your query port and hit scan again
Your server is firewalled.
Please turn off any firewall you have that may block scanning
Your server isn't configured to respond to scan requests.
NOTE: all instructions assume you are not trying to connect from/to "localhost" and are on another IP Address outside your domain.
Open a telnet session to your server on the query port
ex. telnet $IP_ADDRESS $QUERY_PORT
Execute a "use port" command
ex. use port=[myserverport]
Is the result "error id=0 msg=ok"?
Yes, goto 4
No, goto 11
Execute a "serverinfo" command
ex. serverinfo
Is the result "error id=0 msg=ok"?
Yes, goto 8
No, goto 11
Execute a "channellist" commmand
ex. channellist
Is the result "error id=0 msg=ok"?
Yes, goto 10
No, goto 11
Execute a "clientlist" command
ex. clientlist
Is the result "error id=0 msg=ok"?
Yes, goto 10
No, goto 11
SUCESS
Your server can now be been added.
ERROR
Please review the Teamspeak 3 documentation to make the command that failed return an "error id=0 msg=ok" and goto 1