Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
Unfortunately NX420 isn't supported by AirPrint feature. However, you will be able to setup this printer on your Mac via WiFi (not AirPrint). On your Mac, go to Apple > System Preferences > Printers and Scanners. Click + button in there and this will allow you to add the printer to your Mac.
If that didn't work, then download and install the latest drivers on your Mac for printer from here --> http://www.epson.com/cgi-bin/Store/support/supDetail.jsp?UseCookie=yes&oid=151211&infoType=Downloads
(Download the Printer Driver from the above website compatible with your OS X version. Then open the downloaded file and follow the onscreen instructions to add the printer to your Mac)
To print from your iPad (not via Air Print), you can use the Epson iPrint app, which you can download from App Store --> https://itunes.apple.com/in/app/epson-iprint/id326876192?mt=8
Make sure your iPad, Mac and printer are connected to same WiFi network. Or it will not work.
If you have any questions or if you face any issue with adding the printer, please let me know and I will continue to help you.