Where to find the secret key
To find your server secret you will need click "Settings" in your Buycraft control panel then click on "Servers".
On this page, you will see a list of the servers that you have already created. You can also create new servers from this page or edit existing ones to show that server's secret key. After editing the server you will be able to click the Copy button to copy the server's secret key to your clipboard.
Once you have the server's secret key you will need to link your game server with it, the command for Minecraft servers is /buycraft secret <secret key>, however this command differs for different game types.