Android Studio: Newly Created Directory Not Appearing In Folders View
Turns out there is a selector at the top which, by default, is set to show Android. If you flick the toggle down to 'Packages' you can see your own packages.
In Android Studio you can add new directories by right clicking the resource folder in the Android view. (Don't get confused by the New Directory option.)
For example, if you want a new layout resource file and folder then right click layout and choose New > Layout resource file.
If you want a new layout-sw600dp for the activity_main.xml file then choose Smallest Screen Width from the list and click the >> button. Fill in the file name and width and click ok.
Android Studio will automatically create the layout-sw600dp directory and in the Android view you will have both layout files conveniently displayed next to each other.
It is a similar process for adding other resource directories and files (see my other example).
Browse the actual folder structure in windows and you'll see the folder. Put the same resource file found in the normal drawable folder into this new directory. Then when you go back into Android Studio you'll now see 2 files under the NORMAL \res\drawable structure. Make sure you don't look for a subfolder in Android Studio called "drawable-sw600dp" because you won't find it other than in windows explorer. What you will find in Android Studio (after you copy the resource file) is a new resource file along with your original called "jenny-web.png (sw600dp)" or something similar. This same principle resolves the same error when working with other resources like layouts.
Clicking at TAB Android in top, select the perspective Project Files. Then u can see all folders of project. 
For Android Studio 3.0, those who are following https://developer.android.com/training/basics/supporting-devices/languages.html and have encountered the same issue as the one stated in this question, you can also do the following aside from the above answers which are helpful.
- Right click to the target folder (i.e. res folder) > New
- Choose Android Resource File
- Specify the file name, and in the Directory name field put the folder name you want to create or the existing folder you want to put the resource file.
After hitting OK, in the Project files dropdown, you will see the correct strucutre.
And in the Android dropdown, you will see it as part of the strings.xml under values folder.
