Did you try winding the string over itself on the peg? If you're spiraling straight out it's probably slipping off. Wind it over itself so that it pinches down on the peg and holds. I haven't actually messed with these strings myself but that would be my approach..
I saw these put on by tying a knot with a washer at the tailpiece and then they were put on just like any other string in the box. I wonder if heating them up a little (in hot water) would make them easier to turn in the box. Once they are around the pegs , you shouldn't have a problem, but I would definantly tie them. As I remember them, they were just long enough to do the job and no more. Good Luck!
how to put weedwacker strings on an upright bass 
