How to check your Snow Leopard whether boot at 64 mode currently.

1. Mac Logo -> About This Mac -> More info
2. Click on Software heading
3. Check the 64-bit Kernel and Extensions whether for Yes

Method 1:

Hold 6 and 4 keys during boot at Snow Leopard. This method only active one time

Method 2 (Recommend):

Edit this file:

/Library/Preferences/SystemConfiguration/com.apple.Boot.plist

Remark: It must require root permission.

Found this:

<key>Kernel Flags</key>
<string></string>

Change to:

<key>Kernel Flags</key>
<string>arch=x86_64</string>

Method 3:

Open Terminal console type:

nvram boot-args="arch=x86_64"

It is set boot-args to use 64 bit kernel