A QR code is similar to a barcode, but having 2 dimensions means that it can hold more information than a normal barcode.
Websites such as www.qrstuff.com would allow to to use a tool for generating a barcode of the next clue's location. You can use this tool to create your barcode image and then print it off.
When the treasure hunters find the code they'll need a smart phone with a camera to scan the barcode into a QR reader app. Most smart phones have QR reader apps available to install, just search the phone's store/marketplace for QR reader.
You'll need to make sure that your treasure hunters know what a QR code is. If it is likely that they won't know you may have to leave hints or instructions on how to install a QR reader app and how QR codes work.