How do I open an app that Play Store lists as installed, but that isn't accessible?
Here are the instructions from the page that you linked:

INSTRUCTIONS
Setting a live wallpaper works differently from device to device, and on different versions of Android. In most cases you can set a live wallpaper the same way you set a static wallpaper. Try these methods, and if they don't work feel free to contact us:

1) Tap an open area of your desktop, look for an edit icon, and choose "Wallpaper" and then "Live Wallpaper" from the list that appears
2) Go into "Settings" - "Display" - "Wallpaper" - "Live Wallpaper"
3) Go into "Settings" - "Personalization" - "Wallpaper" - "Live Wallpaper

Try going to set the wallpaper the way you normally would and see if WPS Video Live Wallpapers is there waiting for you.