Like others who have posted, I could not get Multiple Uploads to work on the front end on one of my websites. No matter which method was selected, I received a message saying it wasn't installed on my browser. I traced the problem to a plugin I installed to remove JQuery conflicts -- JQuery Easy.
Debug on the plugin indicated that it "removed 2 empty script tag(s)". When I unpublished the plugin, multiple uploads worked. This wasn't a good solution because I needed the plugin to solve some JQuery conflicts. I then discovered that if I changed "Enable jQuery" to "none", multiple uploads worked and only one extension (that I didn't need) appeared to break with this fix. The ones that originally had a JQuery conflict still worked.
I hope this helps others who have (or are having) this problem.