To print from a Mac to an Epson NX400 you need to plug a USB cable into the Mac and connect it to the printer. If the printer does not work when connected you may need to install a driver for your Mac (See links below).
Yes
Yes.. It can!!
Try here. http://www.posmicro.com/RECPRINTERS/EPSON/EPSON%20DRIVERS.htm
There are Epson Scan drivers available for Mac. You can download these drivers online from the MacUpdate website. Once on the page, type "Epson Scan" into the search field at the top of the page and press enter to bring up the drivers.
letter quality
If an Epson Stylus printer will only print the color blue then the other colors of ink have run out. The ink cartridges for yellow, magenta, and black must be replaced.
Replacement Epson Print CDs can be challenging to find. The best place to obtain one would be directly from Epson. The Epson customer service representatives are often willing to send replacement CDs and manuals. Alternatively, a replacement CD could be found through second-hand retailers specializing in electronics.
The Epson Stylus NX415 should be recognised by Mac OS X automatically. Epson have a range of software available for download if required. (See links below)
plug it in a USB port
on a mac you go to file=print
There are many Epson POS receipt printers available for purchase on Amazon and eBay. Epson has their OmniLink receipt printers available, from their website. Some have the ability to print from a mobile phone.
Most epson hardware should work with your mac. Epson has a long history of being compatible with macs; however, I would go to their website and make sure that what ever model you want to use is compatible as apples hd support can be spotty at times.