By default, Mac OS 10.8 only allows users to install applications from 'verified sources.' In effect, this means that users are unable to install most applications downloaded from the internet or stored on physical media without receiving the error message:
"'ALL In Learning Browser Plugin' can't be opened because it is from an unidentified developer. Your security preferences allow installation of only apps from the Mac App Store and identified developers."
To resolve this problem, you'll either need to change your settings to allow you to install programs from unidentified developers or to allow you to install this program and exempt it from the "Gatekeeper."
You can read the full details, including both resolutions in Apple's Knowledge Base Article: http://support.apple.com/kb/HT5290.
However, here are a few quick steps to allow installation of "unidentified developer" programs (for the most recent Mac updates, see below):
Open the System Preferences. This can be done by either clicking on the System Preferences icon in the Dock or by going to Apple Menu > System Preferences.
Open the Security & Privacy pane by clicking Security & Privacy.
Make sure that the General tab is selected. Click the icon labeled Click the lock to make changes.
Enter your username and password into the prompt that appears and click Unlock.
Under the section labeled Allow applications downloaded from:, select Anywhere. On the prompt that appears, click Allow From Anywhere.
Exit System Preferences by clicking the red button in the upper left of the window. You should now be able to install applications downloaded from the internet.
Note: For Mac Sierra 10.12+ users: The process for this has been updated. Instead of "Allowing from Anywhere," once you try to download the plugin and receive the message that it is from an unidentified developer, you will have 2-hour window in Mac to allow the plugin.
Follow the same procedures as above, but instead of Allowing from Anywhere, click to open the file.