have a go at using drumsynth.. its free, easy to use and can create some AWESOME drum sounds, both conventional and unconventional, its also got a decent gui and will help you to understand how kick drums work.

http://www.threechords.com/hammerhead/drumsynth.shtml

If you want to use a synth to make a kick i recommend the maelstrum from Reason V2. Using a sweeping modulation envelop is important.

If you look at how a kick sweeps down in frequency you need to think about what happens when a kick pedal beater hits the skin. It goes something like this..

beater makes contact with the skin. this pushes the skin forward... in a sense it 'irritates' the skin. this takes around 10-20ms in my estimation.

The skin then returns backwards after the beater has started on its return journey.. and its continues to vibrate.

This is the important bit... the freqeuncy of the vibrations are not static, they quickly dive down into the sub territory.

As far as i can remember the 'click' of the kick's first part is high frequencey, and the sub us the end low frequency.

HTH... ;)