To disable the press and hold accented character picker you need to digit the following commad in a Terminal window:
$ defaults write -g ApplePressAndHoldEnabled -bool false
After that you will have to Log Out and then Log In again. Or just restart the Mac.
If you want to re-enable the functionality, you need to digit
$ defaults write -g ApplePressAndHoldEnabled -bool true
the Log Out and Log In again.
