Q: I can iPrint to my Brothers MFC-J410w wireless printer but can't air print.  Why?

That printer doesn't support Airprint.

See here for lists of printers which support Airprint: http://support.apple.com/kb/ht4356

You're confusing wireless or Wi-Fi printing with the AirPrint protocol. All Wi-Fi printers do not support AirPrint. The printer you list does not support AirPrint.

I Just found this:

http://www.brother-usa.com/downloads/allmodels-printing-app.aspx#.Vpe8wzNb2K0

It's a useful list of brother's printers that don't have AirPrint but still you can print through their free app. Depending on your printer model, it unlocks  features you can use using your wifi connection.

Hopefully, in the almost three years since this thread was last active, the OP resolved their issues.

I hope they did, I just posted this in case someone with the same issue googled, same as I did.