Welcome to the Atlassian Community!
Go to admin -> find new apps and search for the app you want to install in the marketeplace.
Or find the app in the marketplace's web site, find the app you want to install, download the latest version that is valid for your Bamboo install, then use Admin -> manage apps -> opload/install to upload the jar or obr file you downloaded.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.