Hi,
have you tried this already:
+*How do I install Rosetta so I can use PowerPC-based applications?*+
+When you try to open an application that was designed to run on a Mac with a PowerPC processor, Software Update will display a message that says "To open <Application Name>, you need to install Rosetta. Would you like to install it now?" When this message appears, click Install to download and install Rosetta.+
+If you cannot use Software Update:+
+Start up your MacBook Air from the MacBook Air Software Reinstall Drive to reinstall Mac OS X.+
+When the Install Mac OS X screen appears, click Customize.+
+Click the box next to Rosetta and click OK.+
+Click Install.+
+To bring your computer up to date, use Software Update after Mac OS X finishes installing.+
http://support.apple.com/kb/HT4399
Stefan