With the Plug, you can earn Plugcoins every time someone downloads an app from on of your Plug links. The amount of Plugcoins for each download depends on where the user is coming from and the mobile OS they're using.
To get an idea of how many Plugcoins you'll receive for a campaign, you can head to the Plugs tab within the Plug. Here, if you tap on a campaign, you can see a full breakdown of how many Plugcoins you'll receive based on where the download originates.
Not seeing Plugcoins registering? Check out this overview on what can cause Plugcoins to not appear within your account.