Looks like linum mode was added to the Emacs distribution in version 23.1 (changelog). Because linum isn't distributed with Emacs 22, you're calling an undefined function and therefore getting an error.
Perhaps you used to run a more recent version of emacs, which has since been clobbered. You could either:
download the linum source, add it to your load path, then
require
itinstall a newer version of Emacs.
Edit: As mentioned in the comments above, you could have multiple Emacs binaries on your path, with different versions. Have a look in /usr/bin
, /opt/local/bin
, et al, to see if this is the case.