Hi Tenpin,
Let me take a shot at this since I wrote the instructions you are quoting. Based on your question, it sounds like you are not using a Consolidated Object Library or you wouldn't be quoting step 5 that is not taken when you use a Consolidated Object Library.
To clarify your interpretation of step 5, let me reprint step 4 and 5 of the installation instructions....
"4. If you have a previous version of the RTMM Object LibraryFSXP3D folder, you will now need to remove it from the Scenery Library and delete it. Then, copy the RTMM Object LibraryFSXP3D folder, included in this distribution, into you main FSX or P3D\Addon Scenery folder area or wherever you keep your object libraries. Note: For "Consolidated Objects Library" users, you will place (with overwrite) the Scenery and Texture files into your "Consolidated Objects Library" instead of copying the library into your Addon Scenery folder area.
5. Add the RTMM Object LibraryFSXP3D folder which is now in your FSX\Addon Scenery folder to the FSX or P3D Scenery Library and enable. Note: This step would not be taken if you use a "Consolidated Objects Library"."
So, in step 4, you move the RTMM Object LibraryFSXP3D folder to wherever you keep your object libraries... Step 5 tells you to now add that folder to the FSX or P3D Scenery Library. In other words, you now execute P3D or FSX and go to the Scenery Library page of the simulator (FSX or P3D) and add the new folder to the library.
I have attached a pic of my RTMM Object Libraries folder (I, too, keep all of my object libraries in one folder) that shows the installed RTMM library folders. Does yours look like that too?
Rod