When I accept a challenge or I do a challenge, I receive the "connect to server" message, but I can't play. Can you help?
Press Alt + Ctrl + Delete key combination and open the Task Manager. Close all the programs that may interfere with GGPO and try again. Also, download and install the latest version of Adobe Air, make sure that your Windows firewall allows GGPO to communicate and try to temporarily disable your antivirus program. You can check the official forum for further solutions.