Q: How in the world do I print double side on Mac?

If the Epson supports duplexing, and you have the correct print drivers installed for it, two sided printing should be an option in the print dialogue somewhere.

What printer model are you using?  Does it support duplex printing?  If it doesn't then you'll have to do it manually.  There are numerous websites describing how:

How to Print Double Sided: 16 Steps - wikiHow

How to Print Double Sided Documents on Any Printer: 4 Steps

This Google search lists many more: double sided printing

Go to the file menu and select Print (the Icon calls QuickPrint)

Is your printer this one?

Very odd doing a search for 402 on Epson's web site matches nothing, but a Google search for "Epson 402" does.

There's a support page here which lists OS X Intel Macs for drivers. I would guess downloading the driver would be what you need.

Since the Epson XP-402 supports Apple Airprint, my guess is that you are trying to print using Airprint.

Airprint is a "limited or no Options" driverless printing system designed for mobile devices like iPhone or iPad.

The current driver for that printer is available for automatic download/install from Apple's server - see this:

OS X: Printer and scanner software available for download

Even though Airprint is available for Macs, it isn't recommended (because of the limitations).

Delete and re-Add the printer in Print & Scan prefs, this time making sure not to select the model name with Airprint in it.

No need to download anything since this driver is available on Apple's server.

If the correct driver doesn't show up when adding, Reset the Printing System to force OS X to re-index "where to find" the driver.

OS X Mavericks: Reset the printing system

What print icon are you referring to? What program are you using?

I just hit the print icon in the Safari's Toolbar and it gave me the print options along with a box, unchecked, for double sided printing. Are you in a different program, where that same print icon in that particular program's toolbar will just automatically print? If so, then you may need to go to File---> Print in that program.

KOT

Even though Airprint is available for Macs, it isn't recommended (because of the limitations).

I have to admit I don't have an Apple laptop - Airprint makes sense if you drag your laptop around and want to print (and don't to install various drivers)!

It would appear that the latest driver would be Ver. 8.69(06-2012) according to Epson's website. Check to see if that is in your printer, if not download it and follow the instructions to install it.

I know that Apple's AirPrint does support double sided printing, as my Canon does it via AirPrint. But that is a matter if your Epson able to do it.

KOT

The latest driver available from Apple servers for auto install is vers 9.0. Don't need to download anything.

Go to Print & Scan prefs, highlight the printer, and click on Options and Supplies, then select Options. Is double-sided available there?

There is another way to set up available options, if it isn't there,  Come back then.

The option to print two sided my not be right in front. You may also looking at the default print dialogue, which is very minimal. Next to the printer name should be an expansion arrow. Click that to see the full range of options. And even that is normally never all of them. Where it shows the selected paper size, that should be a drop down menu of more options.

I just opened a 2011 MS Word document, then went to File---> Print.

On that window click on "Presets: Default Settings" In that drop down menu, you should see an area that states "2-sided printing". Clicking there should enable doubled side printing. This may not be set to do Double sided printing every time, so you may have to select it each time you want to print on both sides.

KOT