Are you looking for a Mac App to mockup iOS Apps? Or a Mac App to Mockup Mac Apps ??
Either/or I have seen a couple of Lite versions, but are very restrictive, but if you Type Mockup into the Mac App Store search, it brings up a few that you are paying a couple of dollars for.
There are some great Mockup Apps on iPad that look so much better than the Mac Apps.
Sorry couldn't be of more help