In order to be able to create a playlist in a station's media library, you need to be a staff member of that station (or, of course, the owner).

Creating a playlist 

Navigate to the station's administration panel and click on the 'Media Library' button at the bottom left in order to access the station's media library.

Select the tracks you wish to add in the playlist and then click on the 'Playlist' menu item on the top-bar. 
Create a new playlist by typing a name for it in the 'Add a new playlist' box and clicking on the 'Update playlists' option.

Your playlist has been successfully created! Click on 'Playlists' and see your newly created playlist containing the tracks you selected.

Now click on the 'Selected' option in the top-bar and select 'Clear all selected items' to clear your selection. If you want to re-order the songs in the list, just select a song and use the 'Move up' and 'Move down' buttons on the top-bar to do so. Make sure you click 'Save Changes' once you are done.

Now that you are all set you can schedule the playlist in the time slots of your preference. Here is how to schedule a playlist