Mask image loaded in UV Editing window doesn't show up under Gaussian Blur modifier mask source in Video Editing tab Basically the title. I've been loosely following a YouTube tutorial where the mask was created within Blender if it makes a difference. The mask was created in a separate image editing program and is a PNG image. Here's the UV Editing window if it helps: 
And here's what I see when trying to use it as a mask:

Any help on how to get this mask image to show up in my modifiers pane would be greatly appreciated!
You have opened the UV editor window

Open image editor window, shortcut Shift + F10
At around top there should be written something View(next to Image Editor button), click it and change it to Mask
You can then create a mask by clicking +     New button.