Q: No music in iTunes Media folder?

The location of your iTunes media files depends on several factors:

• the settings in Edit > Preferences > Advanced, specifically these three:
  
• The method(s) you use to add media to your library.

For example, if you have the Copy files to iTunes Media folder ... option unchecked, and add media (existing files on your PC) using either:

• File > Add File/Folder to Library... or
• Drag-and-drop to the iTunes window

then the media files added to the iTunes library will stay in their initial location, and not be placed in the iTunes Media folders.  There are a lot of possible permutations here: what settings do you have in Edit > Preferences > Advanced, and what issues are you encountering (if any) of media files not being in the location you expect them to be?

With those settings all music that you add will go into the specified media folder structure.  If you want to make sure that all your music (specifically, that added prior to changing the iTunes Media folder location and/or the other two options) is there you need to perform an extra step - just changing the iTunes Media folder location doesn't move anything that was previously added to iTunes and stored in a different location.  To make that change, select File > Library > Organize Library... and when you see this prompt:

click the Consolidate files check box and - if its not greyed out - the Reorganize files ... check box.  Then click OK - this will move any media files that are linked from your iTunes library to other locations to the standard folder structure under the iTunes Media folder,

For more info, see Turingtest2's documents on:

• Make a split library portable - details on how to bring your library into a "well formed" layout for ease of organization as well as creating/maintaining a backup
• Getting iTunes & Windows Media Player to play nicely describes how to avoid issues resulting from iTunes and WMP conflicting over file and folder locations.

(Edit - for media that you've purchased via your phone, you'll need to make sure that all iTunes Store purchases are downloaded onto your PC - see Re-download or transfer your iTunes Store purchases from an iPhone, iPad or iPod to a computer - Apple Support)