Since the Microsoft updates in May 2011, there appears to have been a change which is preventing Dell Optiplex machines from using the PowerOn option within AB Tutor.
Here is the location within BIOS on a Dell Optiplex 380 that allows the remote power on via AB Tutor:
1. Press F2 during boot up to enter BIOS Setup
2. Go to Power Management
3. Go to Low Power Mode
4. UNCHECK Enable Low Power Mode
NOTE: With this setting, the NIC lights should remain on even when the PC is powered off.