Where the Android-SDK is installed depends on how you installed it. If you downloaded the SDK through their website and then dragged/dropped the Application to your Applications folder, it's most likely in /Applications/ADT/sdk (as it is in your case).

If you installed it using Homebrew (brew install android-sdk), then it's located here:

/usr/local/Cellar/android-sdk/24.3.3 # Don't forget to check your SDK version.

Open the terminal window and enter the following (changing out the path to the SDK to be however you installed it):

export ANDROID_HOME=/Applications/ADT/sdk # or export ANDROID_HOME=/usr/local/Cellar/android-sdk/24.3.3

Once you have this set, you need to add this to the PATH environment variable

export PATH=$PATH:$ANDROID_HOME/tools:$ANDROID_HOME/platform-tools