Download the installer from the link above, unzip it, run it and then slide the toggle switch to "On." Enter your Mac's administrator password to allow AirPrint Hacktivator to make some changes, and then you're done. Any shared printer will now show up in the AirPrint dialog on your iOS device. The AirPrint Hacktivator page has a short video that demonstrates how to share a printer if you've never done it before.
While early builds of the Hacktivator did depend on a handful of files from pre-release versions of Mac OS X 10.6.5, the app now uses a different technique for turning the printing pipe back on. The release notes for this latest version of AirPrint Hacktivator include the following text -- "No Apple files are redistributed nor installed by the latest version."
This answers a common question about the legality of all such AirPrint hacks; for at least the Mac version, this application seems to be entirely legal. Reader Ken points out that the controversial Apple-authored files are still present in the package for version 1.7 of the Hacktivator, although they should not be required for it to work at this point. We're reaching out to Netputing to clear this up.
There's also a Windows version available. German website Macerkopf.de has the details on AirPrint Activator for Windows.
I personally went with another AirPrint tool on my Mac -- eCamm Printopia (US$9.99). The feature that sold me was the ability to print PDF files to my Mac or Dropbox. Yeah, I could have done some Applescripting and achieved the same results, but Printopia was just too easy to set up, and it works very well. There's also a third choice that we've discussed here on TUAW -- FingerPrint from Collobos Software (US$7.99). The newest version of Fingerprint adds Dropbox and Mac support, and it also raises the stakes by adding a "Send to iPhoto" feature.
It's wonderful to see so many solutions available for iOS printing pop up to fill the gap left after Apple failed to deliver access to shared printers in the iOS 4.2 release.