Install Far Cry 3 (Reloaded Release recommended)
Download and Install Uplay Client
Download the MP-Fix and extract it to the Uplay Launcher folder
Start Uplay and Register a new Account or Login
Join the Far Cry 3 Network
Start the Game as Administator
Bypass the Serialcode Validation by clicking "OK"
Update the Game
Go to "Co-op" or "Multiplayer"
Join any Match