Wd video don't play

No default application assoociated with the .mp4 file extension. My laptop is a macbook pro and this is the error message i keep getting. See the documentation of your operating system to learn how to associate file types with a particular program. Please help. 

Hello,see if the following steps help.

1- Click one of the files in the Finder.
2- Choose File→Get Info (Command+I).
3- In the Info window, click the gray triangle to disclose the Open With pane.
4- From the pop-up menu, choose an application that you want to use to open the .mp4 file.

5-If you click the Change All button at the bottom of the Open With pane, you will make the application you select the default one for all .mp4 files.