To arm tracks on the CS18AI in DAW mode, simple toggle the "Option" button and then select the tracks you want to "Record Arm." To dis-arm you just deselect the tracks while "Option" button is engaged. You will want to disengage the "Option" button after you finished arming or disarming tracks.
Also, if you have "Audio Record Arm" follows selection, any selected channel will automatically "Record Arm."