In case this helps anyone else, here's my solution: enable Bonjour on the printer.
I have a Epson WF-3620 and after upgrading to Yosemite (OS X 10.10), my Macs could no longer find the printer. I tried every solution that I could find online (repairing permissions, reinstalling and upgrading the Epson drivers, relaxing the CUPS permissions, reset printing system, etc...). I also called Epson Support, which was able to walk me through adding the printer by IP. Since that worked, I knew the drivers worked it was just that OS X couldn't locate the printer. The Epson guy suggested that I try calling Apple support, so I did. The Apple guy was nice, but couldn't really help. He did suggest that I try some more time consuming options including creating a new admin user (to rule out that it was my user profile that was the problem) and reinstalling the OS (in case it was corrupt). I did both of those and neither worked.
After sleeping on it, I decided to poke around in the Epson settings via the web interface. To do this, just enter the printer's IP address in a browser that's on the same network (you may have to skip some HTTPS / certificate warnings). From there I looked around until I found Wi-Fi/Network Settings / Services / Protocol. On my system, Use Bonjour was unchecked. I had seen in other people screenshots and videos that when the Add Printer tool works, it shows that it connects via Bonjour Multifunction, so I enabled Bonjour and saved the settings. After waiting a few minutes for the settings to be applied, I was able to add the printer via the Add Printer Default tab (I did change the "Use" setting from "Secure AirPrint" to "Epson WF-3620 Series" option -- I think this is the Epson specific driver).
I don't know if Bonjour is disabled by default or if I turned it off somehow in the past and forgot, but it's an easy enough fix that might work for someone else so I thought I should share.
Good luck!