Check your System Preferences before downloading !
IMPORTANT: Your System Preferences might need changing before you can download as Apple has set up newer versions of OSX to only install Apps from their App Store (Apple take a 30% cut of all sales from their App Store).
To change this go to 'System Preferences' in the Dock (as icon above) and then select 'Security & Privacy'. Now click on the padlock to the bottom left to make changes and enter your password. Where it says 'Allow apps downloaded from:' select the 'Anywhere' option and click the lock to finish. After installing iA you might want to go back and reset your preferences.
Check System Preferences then continue...