Hi Monica,
After reading the thread completely, I can agree with you, it is quite chaotic. There they talk about the rj_insertcode editor plugin, but as things appear to have changed a bit since Moodle 2.4, the instructions given do not seem to work. I tried to follow them but all I accomplished was to completely vanish the editor area 
A few have managed to have it working, though they don't explain how they did it. As it is still being developed, I suppose it won't be long until they create a page with the final plugin and instructions for each Moodle version.
Regarding the error from the Geshi filter:
$geshi =& new GeSHi($source, $language);
you only need to delete the ampersand to supress it. I also tried this, but code was simply displayed without any fancy attributes.
I do have some good news, however, the TeX filter works Ok. I just enabled it at the text filter plugin page and then added some simple code in a page resource; the image rendered fine, and this was based only in the Mimetex provided by Moodle.