Ok.
I found the solution.
The new iTune (v11.0.4.4) doesn't allow you to directly add mp3s to your iPhone.
Rather, it wants you to "add" songs to iTunes first, then, use the iTune-added songs to iPhone
(PC -> iTune -> iPhone)
Now,
1. Ctrl+B to show Menu Bar.
2. Don't go into your "iPhone" yet. Go to "Songs" on the top menu below the Menu Bar
3. File->Add File to Library -> select your songs to add to iTunes
3.1 Organize your songs into playlist (optional)
Now, you have added songs to iTunes. Now, time to add it to your iPhone.
4. Click "iPhone" in the right upper corner
5. Go to Music and select "Sync Music"
I personally don't select "Enire music library" but select "Selected playlists, artists..." becuase I like to organize songs in playlists.
5.1 If you select "Selected playlist, artis,..." Check (in the checkboxes) the songs that you want to add to your iphone
6. Click "Apply" in the bottom right corner.
Done!
A comment to you Apple... YOU SIMPLY SUCKKKKKKK