Change characters in press and hold popup? Seems changed in Yosemite?
Pressandhold.app should be in exactly the same place in Yosemite that it was in Mavericks.

I just checked on both my Macs that I upgraded to Yosemite.

/System/Library/Input Methods/PressAndHold.app exists exactly where it should.

And to try to confirm whether this is a holdover from Mavericks (since both my Macs were upgraded from Mavericks to Yosemite) I checked the Info.plist with the PressAndHold.app bundle. It lists "10.10" as the LSMinimumSystemVersion and "macosx10.10internal" as the DTSDKName both of which indicate Yosemite.

So please check that folder on your Mac again. I'd be very surprised if that folder was empty.