At the top of the mission editor screen you will see some options. One is called: Groups (F2). Either click on it or press F2.
Now find the marker - which will not be too easy as it is now invisible. Click on it and drag over to the unit. then let go of the mouse button. You will now have a light green line connecting the unit to the marker. That is what I meant by group each marker with the unit.
An easy way to do this is to put the marker near the unit, then when you have grouped then together, go back to the Markers option and move the marker to where you want it.