try run
android update sdk -u
in terminal.
You will see logs like this on your screen
Installing Archives:
Preparing to install archives
Downloading Android SDK Platform-tools, revision 17
Installing Android SDK Platform-tools, revision 17
Stopping ADB server succeeded.
Installed Android SDK Platform-tools, revision 1799%)
Downloading Android SDK Build-tools, revision 17
Installing Android SDK Build-tools, revision 17
Installed Android SDK Build-tools, revision 1799%)
Downloading ARM EABI v7a System Image, Android API 17, revision 2
(71%, 775 KiB/s, 41 seconds left))
After Android SDK is successfully updated, make sure Build Tools is installed.