Along with several small fixes in the recently recently released 10.8.1 update to Mountain Lion, the tech boffins over at Cupertino managed to ‘break’ the purge command. Now, as this isn’t a widely advertised OS X command, a fix should be along in the next OS X update all being well.
In the meantime, should you need to use the command to free up memory, you can use terminal to create a ‘soft symbolic’ link. Although this tricks OS X into allowing the command to be used, it’s not a permanent solution. Thus meaning it will be overwritten with the next update.
To do this, fire up Terminal then type or copy and paste the following commands:
sudo ln -s osx-12.0.0.xml osx-12.1.0.xml
You may be prompted for your password, if so enter it as normal into the terminal window, bear in mind nothing will appear as you type your password.
Once done you will then be able to type purge into terminal to free up memory resources once more.