I put settings in a file called ~/.Xmodmap:
xmodmap -e 'keycode 66=Return' xmodmap -e "clear Lock"
The next time I logged in, I was prompted as to whether to load ~/.Xmodmap. I loaded it, and tick “don't ask me again”.
Still it does not remap the desired key. I am using Linux Mint 13 (maya).
The commands work perfectly if I run them manually.