Howdy! I had the same problem when i got the kit initially. The ever helpful Mr Gary Stark replied to my PM with the following:
Yes, the first couple of times these can be a bit of a challenge.
I'm going from memory here ... first of all, you need to mount the mounting ring on the light head. Note that there's an optical trigger on the top of the hear. Youshould not cover that with the softbox.
There are four holes, at 90 degree intervals, along the outside of the ring. You also have, for each softbox, four flexible rods, as well as the actual reflector. You'll see that the reflector has three pockets and one velcro tab. These are where the the reflector mounts onto the rods ... shortly.
Slip the reflector over the ring onto the head, again making sure that you're not going to cover the optical trigger.
Now start with the rods: you'll see that there's a sort of bulb on each end, but you'll also see that they're different sizes. One of those "bulbs" will fit somewhat comfortably into the holes in the mounting ring. Mount them onto the ring, and then for each of the first three corners, slip the other end of the rod into one of the pockets on the reflector.
The rods should bend to allow you to do this, and they will start to form an arc INSIDE the reflector as the reflector is stretched over them and out, in front of the light.
Next do the same with the final rod and the final - velcro - corner, opening the velcro and securing the rod into a sort of custom pocket with the velcro.
You should now be seeing the basic form of the softbox, and should be able to mount the diffuser over the square opening you've created.
Done!
The biggest problem i had was worrying i'd bend the rods to far and break them, but you really can bend those suckers a long way.
Let me know if you need anymore help!