When I try to play multiplayer over WormNet on worms armageddon, and try to enter a game, it says "ERROR: Could not get CheckSum 15." Does anyone know what that means or how I can join a game???

Re-install game

Apparently,

"This occurs when the game has been improperly updated."

Try installing the latest update (http://www.nanacide.com/wahelp/WA_update-3.6.26.5_Beta_Installer.exe) again and see if that does it. If not, reinstall then update.