just like Gregory Seront said here:
Actually if you downloaded the icons pack from the android web site, you will see that you have one folder per resolution named drawable-mdpi etc. Copy all folders into the res (not the drawable) folder in Android Studio. This will automatically make all the different resolution of the icon available.
but if your not getting the images from a generator site (maybe your UX team provides them), just make sure your folders are named drawable-hdpi, drawable-mdpi, etc. then in mac select all folders by holding shift and then copy them (DO NOT DRAG). Paste the folders into the res folder. android will take care of the rest and copy all drawables into the correct folder.