In Service Studio select menu Environment then Open files... then on the dialog displayed change the file type from OutSystems Module Language to OutSystems Application Package, see screenshot below, and select the .oap file that you downloaded in this thread. This will install the application in your environment.

Alternatively you can open service center go to factory > applications and below the title there is a link "publish an application". You just need to choose the .oap and 1 click publish.