How do I re-enable the Press and Hold character picker on OS X Lion?
The command to enable the Press &amp; Hold character picker is as follows:

defaults write -g ApplePressAndHoldEnabled -bool true


Note the addition of defaults write which was missing from your command.



If it does not work in certain apps after performing this, restart/reload those apps (e.g. killall -HUP Finder). Alternatively, log out of your user or just reboot your computer.