Play button is only avaible if there is a selected profile with files to play. To play a file simply click on PLAY button. If there is already selected file, app will play it, otherwise it will play random file.
If you want more control over played files, then use PREVIEW button. It will randomly select file from collection of files for selected profile or if there is any selected folder or file extension, it will use this selection as further filter for selecting file.