Throw your PowerPoint rules out the window. Creating a new slide in Showcase is a different beast.
Follow these instructions to add a new slide to an existing presentation — but don't forget to link it to a hotspot so Viewers can see it!
In the web app, open the presentation in the Editor.
On the left-hand side, find the thumbnail of slide you want the new slide to come after, and click the 'More' menu (the three dots). Then click 'Add slide'.
Select 'New image slide(s)' if you already have a slide background image ready to rock, or 'New blank slide' to start from scratch.
This will create a new slide, which will appear in the Slides list in the left-hand column. Edit your new slide! Try adding a background and creating some hotspots that link to files, videos, or other slides. Go bananas!
Note: Don't forget to hit Publish so everyone can see your beautiful new slide!