Have a picture, or can you find a picture of something that looks like your pad? I may be able to help you figure out how it works.
Bareback pads are going to slip no matter what, because they have no support structure. I reccomend mounting from a fence or other high place. Don't use the stirrup (I personally don't like stirrups on bareback pads anyway); just hop up there without it.
Sorta like this, only have someone hold your horse and do it from a mounting block without the running start .

Unless you really want to try the running start. It's fun stuff.