Canon 400 5.6 and manual focus

[Methodical]

I have the 50d and the Canon 400mm 5.6 and tried it with the 1.4 tc and the tape trick but the autofocus was jittery; works sometimes and somtimes not. So since the tape trick is not consistent I need to get better and faster at manual focusing.

Is there a setting in the 50d that will allow me to use the auto focus alert flash thingy (not sure of the name) in the viewfinder with manual focus? In other words I wanted to know when manually focusing how can I get the little focus alert thing to flash that way I know I am in focus while using the 1.4 with the 400 5.6. I hope this is clear.

Thanks