When you go to “Plugins->Add new” you are adding a folder to your plugins folder within WP-Content. You can also upload this folder manually. I generally choose to upload all my new WP plugins, themes etc. manually, because my host’s FTP is rather slow, and has caused issues with the WP’s automatic methods in the past.
I tried doing exactly the same thing on another blog with a current version of WP on it, and everything worked fine. (What I upload is the un-zipped contents of the zip file created by your application.) I also added a second slideshow from within the plugin (letting it do the upload itself this time) and that also worked fine(on my newer site).
Thanks anyway though.