I have created a schedule in just:play but video files (or graphics) which I have placed in the timeline don't show up in playout.

Check if the files have proper access permissions. You need to have an administrator account and the files must be accessible by just:out. To check/change the permissions of files use the "Get Info" command of a file. You can also use the "chmod" command in the Terminal (only for advanced users).

Check also if the layer you are using is activated or not. just:play and just:live comes per default with 3 layers (1 video 2 graphics) activated. 

Please make sure that the engine client and the interface client set for this channel can "see" the folders defined as the repository folders. Make sure those folders are mounted on the machine running just:out, in case such folder is stored on a network based storage. Drag and drop the repository folders into a Terminal window to verify verify the repository folders name and path.