To enable the 64 bit kernel, hold the 6 and 4 keys while booting. Likewise, to enable the 32 bit kernel (if it is not your default), hold down the 3 and 2 keys while booting. By default, Snow Leopard has the 32bit kernel enabled to provide better application support until developers have ported their applications over to 64bit.

To permanently make the 64bit kernel your default enter the following from a Terminal session.

Update:
This didn’t work for me.
sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.Boot ‘Kernel Flags’ ‘arch=x86_64’

However, this did:
nvram boot-args=”arch=x86_64″

To verify the kernel you are running, click on the Apple symbol in the top left corner and client “About this Mac.” Then, click “More Info…” and then Software. Look for the following:

64-bit Kernel and Extensions: Yes

If running the 32-bit kernel, it would read the following:

64-bit Kernel and Extensions: No