Following Jeff's tutorial, Go back one step by step. The scenery file that you downloaded for the new Misty's Place is a compressed zip file (Misty's Place v4.6.zip) so you have to unzip it before it is usable.
Unzip that zip file using winzip or winrar etc and then you'll be able to see the new scenery files inside the main folder. That scenery folder contains all the new scenery files for the new version of Misty's Place that needs to go into your add-on scenery folder "Misty's Place". (just make sure to delete the old Misty's Place scenery folder then replace it with the new scenery folder from the latest version of Misty's Place v4.6.
Then delete the old scenery folder and replace it (drag/drop or cut/copy/paste with the new scenery folder/files. Start your sim and it should refresh the sim indexes for the new scenery files as long as you don't change the scenery folder name from Misty's Place or whatever you have it currently named or set up in your sim.
Almost every scenery file download from RTMM works the same way. Comes as a zip, unzip it, copy/paste/replace the required folders/files and set up the scenery in your sim scenery library if has not been setup. Done.
If your still stuck, can give you a hands on hand via teamviewer if need be. Let me know via email and can arrange.
Norm