Under Windows XP they are located at C:\Documents and Settings\Your User\Application Data\Adobe\Lightroom\Develop Prests\WOW PhotoPresets.

Under Windows Vista Under Windows XP they are located at Local Disk (C:)\Users\Your User\AppData\Roaming\Adobe\Lightroom\Develop Prests\WOW PhotoPresets.

For Mac Users:
Mac HD/Users/User Account/Library/Application Support/Adobe/Lightroom/Develop Presets/WOW PhotoPresets