As you have found out, the DCP-T420W is not an AirPrint model, and the Brother download site seems to suggest iPrint&Scan. CUPS drivers on the other hand are offered for the DCP-T510W model (see my previous reply above). Unfortunately, I do not have access to these printers, but dooooooom appears to have tested it (I can only assume, with a DCP-T420W). A CUPS driver should allow you to print directly from other applications.