I guess you don't have a Desire Z/G2. I'm pretty sure you have a Nexus One or a Desire
In the latest releases the miui developers added a check for the trackball wake option.
Only on bravo and passion the option is displayed. I found a solution to patch the Settings.apk to show the option on vision too.
The followin lines have to be added to Settings/smali/com/android/settings/KeySettings.smali after line 280: