So, you've double clicked on the little blue icon to add a new slide style. You get that little message that says your Slide Style has been successfully added. Great!
But, when you go to look in your Slide Style List you can't find it anywhere! Where did it go?!
The answer is that it may be hidden.
To see ALL of the Styles in any category (or in All Categories) make sure that the Layers drop down is set to "All Layers" and your new Slide Style should magically reappear! You may also want to change the Aspect Ratio to "All Ratios" to rule out a Ratio mis-match.