First, I assume we are talking about the TA, offline or in H2H where you can control the settings.
To enable the Lead Computing Gunsight you goto Setup > Arena Settings and Flight Mode Flags and hit change. Check the Lead Computed Gunsight box and exit out.
Once that is done to see it inflight you hit 'control-tab' to enable Friendly Lock. Once enabled a 'tab' will cycle through targets in your view or a 'shift-tab' will highlight the centermost target in your view.
Highlighting a target like this is also the aquisition mode for the padlock view also, I'm pretty sure.