Save the Mac App Store Password for Free Downloads in OS X

How to Allow Free Downloads from the Mac App Store Without Password Entry

1. Quit the App Store on the Mac if you have it open
2. Open System Preferences from the  Apple menu and go to “App Store”
3. Under “Password Settings” click on ‘Free Download’ and choose “Save Password”
4. Under “Purchases and In-App Purchases” select either “Always Require” or “After 15 Minutes” is what the header question “Require a password for additional purchases after a purchase from the App Store with this computer?” pertains to, choose whichever is appropriate for your Mac usage
5. Leave System Preferences and relaunch the Mac App Store

Source: Save the Mac App Store Password for Free Downloads in OS X