Hi,
To control an aircraft, whether by voice or by mouse (the latter way existing only in solo mode with voice recognition turned off), you need its
callsign. So you must first figure out a callsign for the aircraft.
To control by mouse on radar, you must also have a strip linked to the radar contact.
NB: Whether on a strip or in a radar tag, "?" means that the callsign is unknown. Blue background is the default colour for an assisted identified link.
Have you read the
user guide about linking strips (current §1.1), and watched the
video tutorial (part 2 explains strips and links)?