Question about Cavalry Archers and attack modes

[ReluctantSamurai]

the command to have my cavalry archers to attack/charge

Press Alt+click....that is, hold the Alt button and use the mouse to click on the unit you wish them to engage in melee with. If you want them to charge, dbl. click quickly on the unit you want them to engage.

As a note, CA don't have a very high charge bonus (it's 3 IIRC), so don't expect them to impact an enemy unit the way a Yari/Heavy/Nagainata Cavalry would...