My iMac has OSX lion and cannot be upgraded further. How can I transfer iWork documents to my new MBA using mountain lion?
Wouldn't this be the same as sharing any other document?
In Numbers, hold down the command (⌘) key and click the proxy icon in the title bar. Clicking the name of the folder that appears in the list immediately under the file name will reveal your document in the Finder.
Then, using the Finder, you could transfer the document using File Sharing, a USB drive, as an attachment to an e-mail.
If both Macs are on the same network, you can use either AirDrop (which I personally dislike due to its interface and quirks) or Messages - enable the Bonjour account on both, then you can drag and drop files back and forth.
You can also enable File Sharing in the Sharing section of System Preferences, which is a bit more complicated but offers much more flexibility.