There's a couple things that usually work for getting the steering wheel off. First start the nut back on the shaft until the threads are close to flush with the top of the nut. While pulling up on the steering wheel hit the bottom with a rubber mallet.

If that doesn't work with the nut started on the threads sit on the seat and put your feet on top of the dash and knees under the steering wheel. While pushing up with your knees hit the nut with a hammer and a brass drift. 

Make sure you have the nut started on the shaft to protect the threads and your teeth when the wheel pops free...

Also it's a good idea to spray some penetrating oil on the shaft and let it do it's thing...