In the top-left corner, click Games -> Activate a Product on Steam .
If you have landed on this page, you are likely a dedicated fan of the Arma series, a modder looking to play the legendary DayZ mod, or a tactical shooter enthusiast trying to breathe life into an older purchase. The search term "Arma 2 Reinforcements CD Key Updated" is highly specific, and for good reason. The activation process for this decade-old expansion pack has become notoriously tricky on modern systems. arma+2+reinforcements+cd+key+updated
If you only want the BAF and PMC single-player campaigns, consider buying Arma 2: Combined Operations (which is still on Steam) and then purchasing the and Arma 2: Private Military Company DLCs directly. This often works out cheaper and is 100% legitimate. In the top-left corner, click Games -> Activate