I got the same problem. 0.1 rc2
It does recognize the <pre> and apply line numbers, but the color highlighting doesn’t work.
Yes, the css is loaded correctly, but the code itself gets no classes applied that would do the coloring.
Oh, and line numbers are on by default (not false like it says in the docu).
EDIT:
Ok i’ve found the problem, as mentioned by Frost, you’ll need to download the package from GeSHi website.
http://qbnz.com/highlighter/index.php
Inside there’s a "geshi" folder containing all the language definition php files. Uploaded that "geshi" folder to the /core/components/geshi/lib directory and the coloring now works.
Why aren’t those included in the modx package? Since they’re essential for the extra to work correct.
Thanks anyway for your time and the nice extra.