1. How to find it
Open Android studio, Goto Android Studio>Preference Search for sdk And something similar to this(This is a windows box as you can see) will show
You can see the location there, most of the time it is /Users/<name>/Library/Android/sdk
2. How to Install It,If not there
Just go-to Android Standalone sdk download page , download the zip file for OSX and extract it to a directory
3. How to add it to the path
Open your terminal application and open paths file in nano by typing
sudo nano /etc/paths
Input your password,Go to the end of the lines and input the directory path. And you want to add: eg:-
/Users/username/Libs/android-sdk-mac_x86/platform-tools
Save it by pressing Ctrl+X, Restart the terminal application and To see if it is working or not - type in the name of any file or binary which are inside the directories that you've added and verify it is opened/executed