Installing Android SDK -

1) Download the "SDK Tools Only” Mac OS X from here: 

2) Unzip the folder, and move it somewhere appropriate (e.g. ~/Development/) 

3) Run the android-sdk-macosx/tools/android file (this launches the SDK manager)

4) Using the SDK manager install: 

Android SDK Tools

Android SDK Platform-tools

Android SDK Build-tools

Android 5.0.1 (API 21)

This will download the necessary files into the folder you just unzipped.

5) Now open IntelliJ, select create new project -> Android -> Application Module next, next until you reach this page page (without the Android API selected): 

6) Select New.. and then select the folder you unzipped previously. IntelliJ should recognise that this folder contains the android SDK and allow you to select it as the current projects SDK as can be seen in the screenshot above. 

IOS Setup -

1) Install Xcode - 

2) Create a simple app and deploy it -

Further Reading - 

For a more detailed introduction to setting up your IDE for app development check out David Luna's blog entry here: