Similarly to assigning jukebox credits to a song to queue up its play, applying Play Credits to a song in your Radio Airplay account is what actually queues up the song for that many plays in Jango Radio stations.
To apply your Play Credits, first sign into your account from HERE.
Once signed in, place your mouse/pointer over the PLAY CREDITS tab in your account's top menu. You'll then see USE CREDITS in the drop down section - click on USE CREDITS.
From that USE CREDITS page, you'll see if you have any available credits for use marked as "Play Credits Unapplied" and if you have any credits currently Applied to your song(s).
Place your mouse/pointer over "Apply Credits" next to a particular song on your list, and you'll then see a list of amounts appear - choose one of those amounts to place that many of your unapplied credits to the song - this is what will actually queue up the song for that many plays to Jango Radio stations and the listeners there.
Once credits are applied to a song, the new plays will begin automatically within a few hours. You don't have to do anything else. As the new play credits are used, your Radio Airplay account will update with information about those new Plays and Listeners - Each used Play Credit will correlate with a new Play and "Recent Listener" in your account information.