I've got the SD450, but I'm assuming the controls are the same as your SD400...
You can set the flash exposure lock by setting flash to "on", hold the shutter button down half way and focus on your subject, NOW also (yeah hard to do!) press the ISO button and the flash will prefire and lock the exposure. Any subsequent shots will NOT fire the preflash because the setting is locked in the camera. If you press the ISO button again or change zoom you will cancel the flash lock.
My point here is that by using your existing slave and locking the flash exposure you should at least be able to get the slave to work and see if it makes your photos any better.
I may have missed a step or two in explaining how to lock the flash exposure setting, in that case double check with the manual. Let me know if it works as I'm thinking about getting one also.