I think this has come up a number of times before. The basic beacon that ADE places does not work. It may be that there is a sim object or similar which will work - what have you tried?
Is there a working beacon that can be added via Dev Mode with ADE in Two Way Editing Mode?
There was a response in post #49 about how the xml looks in the Dev Mode. The beacon has to be attached to the tower as a part of it.
<!--SceneryObject name: RotatingBeacon_Tower (Beacon)-->
<SceneryObject lat="41.25707220959646" lon="-70.06336094234626" alt="-2.99999999318350" pitch="0.000014" bank="-0.000014" heading="-179.999991" imageComplexity="VERY_SPARSE" altitudeIsAgl="TRUE" snapToGround="FALSE" snapToNormal="FALSE">
<LibraryObject name="{A04B772A-6BBF-4103-B263-D479BCCE71F5}" scale="0.100000"/>
<AttachedObject attachpointName="" instanceId="{B9BBA9EB-6AD1-4084-9EE8-58CEC5366330}" pitch="0.000000" bank="0.000000" heading="0.000000">
<Beacon type="CIVILIAN" baseType="AIRPORT"/>
</AttachedObject>
</SceneryObject>
It might work if you generate this xml when you add a Beacon from the menu. I have not tried Two Way Editing because I am not familiar with Dev Mode. I tried it once before ADE and gave up trying to figure it out.