Download WikiTeX and extract it to tiki/lib/extensions/wikitex/. Then put the file attached to this page in tiki/lib/wiki-plugins/.
WikiTeX need latex to work, read WikiTeX's readme to know what you have to install to be able to use the extensions you want.
Be sure that your php configuration allows you to use the php function shell_exec. If you are in safe_mode you can substitute it by exec (in wikitex/wikitex.php) and eventually move wikitex.sh the script to be executed in your safe_mode_exec_dir directory. And make sure that php can write in tiki/lib/extensions/wikitex/tmp/.
You may want to put an index.php file in tiki/lib/extension/ and in tiki/lib/extension/wikitex/ like the one you find in tiki/lib/ and a .htaccess file containig
deny from all
to deny access, but you must allow access to tiki/extensions/wikitex/tmp/ by putting in it a .htaccess file like this one
allow from all
If there are extensions that you don't want to use you can comment them out from wikitex/wikitex.inc.php
You can also write your own extensions to WikiTeX by following it's documentation.
Beware that using latex may cause security issues - read wikitex's doc!
In effects ))WikiTeX(( limits the commands that can be passed to latex.
Also note that wikitex mantains his own cache of images in wikitex/tmp/ (you may want to delete them sometimes)