1. put the sprocket over the cranks all the way up to the lip.
2. put the spindle facing down in a vice so that the sprocket is at the top of the vice and the spindle is facing out the bottom
3. make sure the bolt hole is lined up.
4. wack the top of your crank so it pushes onto the spindle....
I had the same problem with my cranks, although i wasn't running a black lotus sprocket, the fitting on the wombolt is a really tight fit, but that seemed to work for me pretty well...
either that or you have a 19mm sprocket