Many systems have multiple instances of the php.ini file, and it can be confusing which one to trust.
The best course of action is to implement a test page under your site that simply calls phpinfo().
Search the output of that page for "mbstring" module.
If you don't see it there, it's not installed or is not enabled by default in PHP.
Please read the manual for installation details:
http://www.php.net/manual/en/mbstring.installation.php
Cheers!