First, make sure "Stitching" is turned on. To do this, click the gear icon next to the title and set "Stitching" to on.
Then, when you click the "Add Slide" button, a new dialogue pop-up will appear. Click "ADD" under "Stitching," then a window will appear.
Click on the slides you wish to insert into your Buncee, then click "INSERT."
The slides have now been inserted into your Buncee.