Step 1 : Move from Android view to Projects view
Step 2 : Now under app folder you should see libs, if you don't see it, create it
Step 3 : Drag & Drop the .jar file here, you may be get a prompt "This file does not belong to the project", just click OK
Step 4 : Now you should see the jar file under libs folder, right click on the jar file and select "Add as library", Click OK for prompt "Create Library"
Step 5 : Thats it!! Now this jar has been added.
Note :We do not need to add the dependency in build.gradle manually unlike previous versions of Android Studio. But to verify, just move to the Android Project View again and check Gradle Scripts -> build.gradle , you should be able to see the the jar added under dependencies , nineoldandroids-2.2.0.jar is the jar that i have added.
compile fileTree(dir: 'libs', include: ['*.jar'])
Note that the process to add an external Project Folder to an Android Studio Project is not the same.