You gotta make sure that none of the folder names have spaces in them in order to play the videos.
Viewing post in Retro YouTube Simulator comments
No folder names leading to the game directory should contain a space. For example, the directory "C:\Users\Username\Desktop\Misc\YoutubeRetro" have no folders with a space between them.
Where people cannot play the videos is when any of these folder names contain a space between them. For example, the directory "C:\Users\Username\Desktop\NEW FOLDER\YoutubeRetro" would not allow the program to playback videos, because the folder name "NEW FOLDER" contain spaces.