Currently it is not possible to add footnotes/endnotes in Impress slideshows (actually, I didn't find anything about how to do it). I would have liked to have that feature to add endnotes in the last slide (like a bibliography and/or references). But having footnotes would also be a interesting feature. Ideally I even would have liked to use Zotero [1], but zotero right now seems to work only in Writer. Tested on Linux Libreoffice 4.3.3.2, but I guess the feature doesn't exist on any OS [1] http://zotero.org
Bug 80674 is a request that to my mind would require adding reference capability. I'm closing this and you can comment on 80674 with your thoughts. Btw. here are Zotero's comments: https://forums.zotero.org/discussion/34976/openofficelibreoffice-citations-in-impress-and-draw/ *** This bug has been marked as a duplicate of bug 80674 ***