Although you have mentioned having no problem placing the light, here is a rundown of all the steps that worked for me, in case anyone else may benefit (these tips were shared by helpful fellows on this here forum ):

- Create the light itself as a separate project, a box of about 1 foot square works well. Under Part Properties, create a new Material colour, you can name it light_red, with values in all colour boxes (Diffuse, Ambient, Specular & Emissive) at 255, 0, 0, 255 respectively.  Do not apply any textures.
- For a steady light effect, simply merge this with an existing project. If adding to a scenery object already in the sim, you can determine it's placement by making adjustments to the LAT-LONG figures in the Project Properties>Area Block field. Best way to find the LAT-LONG and altitude coordinates is to put an aircraft into slew mode to facilitate placement in the exact location. Another way is to make the part grossly oversized, you can then scale it down once the correct location is obtained.
- For a blinking light effect, select Part Properties, and click on the ADD button below the Pre-Process window. Then click Condition>Load Preset and select Beacon On. Check out the other options under Pre-Process to see what else can be done, e.g., lights can be displayed only at night.
- To create the actual object as scenery, click on File>Create FS Object File>Object File (.bgl).  Save it wherever you keep your addon files and it should load next time you fire up FS.