This has cropped up from time to time, so I'll pin the solution here permanently.
You just need to change the process a little when you add a scenery folder with Windows 7. Now, when you select the scenery folder and press 'OK', the scenery folder will open. Just click any BLANK area within the scenery folder (just under the listed 'Scenery' and 'Texture' folders). That's all -- you should return to the Scenery Library with your new scenery folder active at the top of the list.
This link doesn't seem to work at the moment -- try here, it should be at the bottom of the page.