Well I guess you would have to ask TGC to answer that specifically. Perhaps they will. There have been various posts regarding the reasons why it was dropped out. Obviously the product never reached its target functionality.
Of course as both the promised segment and entity editors were dropped from the product I guess the manual could and should have been changed to remove the references to the missing .exe's
As to how to get your entities into FPSC - just open some of the default entities script files in notepad and look at where the various components are kept. Make your entity model in a modeller and save as an .x file - make a copy of any default entity files you want to have a similar entity to - rename them then make get your new x.file model and a texture for it - a bitmap icon for it and the script file for it that will all now have your new entity names referenced as you should change them accordingly - put the various files in the samne locations as the default ones you copied. Then in the editor you should see your new entity icon shown in the entity listngs - just click on your new entity icon and drag one into your level - you should see it appear. Thats all - no importing needed or possible.
Thats all - if you dont want to make new entities you can use default entities and just make new icons, script files and textures for them. If you want to use a default entity then you wount have to make a new .x file model just use the default one. e.g. for a simple box or crate one x file will do for all instances.
Hope that helps a little.