Carl,
In a slideshow, as phroze describes and you might have already experienced, the hidden tags go to the top (or sometimes to the side) when they are not displayed. Pagelime sees they are hidden so puts them off to the top or side, there is no way for them to totally become hidden and unhidden.
You can however add entries to your slideshow by using repeatable regions.
Say you have a list based slideshow as most JQuery sliders/carousels are-
Simply put the cms-repeat tag on the last "li" item tag in the list. (On some slideshows you might have to put the "cms-editable" in addition to the "cms-repeat" class tag on the "li" to get the tag to show up, because of a bit of JS funk)
If you want to be able to order your entire slideshow- create a slideshow with only one li item that has the repeat tag and populate the slideshow within Pagelime, then you will be able to use the "promote" "demote" arrows to order the slides.
I hope that made sense. If not, feel free to email me adam(AT)pagelime.com, I will also be posting a Pagelime 101 video to the blog on this topic this week about repeating-regions, showing how to do what I'm talking about.