I don't know the answer your question. However, my suggestion is
not to use landmark markers, even if you find a way to turn them on. It is much more fun to recognize them visually. For that, the high resolution elevation and surface textures of Moon (LOD12-13, downloadable from Orbiter's main site) need to be installed. Also you will need a high quality map of the Moon, for that purpose I use the
LROC Quickmap. On this, you can easily find landmarks by lat-long coords. Please be aware that in Apollo days, a bit different selenographic coord sys was used, so the landmark coords what you find in the original docs (e.g. flight plans), are off with a few hundred meters. In the quickmap, you can also see the names of the landmarks, just turn on Layers->Overlays->Nomenclature. With this method, I am usually able to find a landmark in the sextant if it is larger than circa 100 meter.