"Mockup" isn't a PowerPoint term/feature. A mockup is just a non-working
illustration of what a proposed design looks like.
In PowerPoint, it'd be a drawing of what your interface will eventually look
like. If it's a system with buttons and such, you could also assign action
settings to the drawing pieces in PPT to make the mockup more interactive, if
you like.
Now if you want to make a mockery instead of a mockup, just mention food (or
heaven help us, worms) and stand back.
