I have the same setup, exactly, that he has. I have a D12-100 receiver. Here is my process:
1. set out antenna
2. turn on antenna controller
3. when lights stop flashing, press scan button
4. when lights stop flashing again, turn on directv receiver.
Works every time.
NOTE: If his receiver is SWM only, it will not work with the Flex without other hardware. You would have to add a SWM8 and PI (Power Inserter) between the receiver and the antenna controller. Note that it can not be between the antenna controller and the antenna, also, you will have to run 2 coax cables from the antenna to the SWM8. If his receiver is SWM only, it is easiest just to buy a receiver that is backward compatible with the old multiswitch technology. That is what the FLEX uses. If I recall correctly, that means HR24 or lower.