If that doesn't help ...
- Apps such as Little Snitch, Growl, and MacKeeper can prevent apps from downloading
- Disable anti virus software if installed
- Make sure the firewall is turned off in System Preferences > Security & Privacy > Firewall
- Make sure Gatekeeper is enabled
Open System Preferences > Security & Privacy then select the General tab.
Make sure either Mac App Store or Mac App Store and identified developers is selected.
If that area is grayed out, click the padlock icon to proceed.