Have you followed the documentation here:
http://php.ca/manual/en/mysqli.installation.php?
Looks like you're on windows, so you'll need to add the libmysql.dll to a system path if you have < 5.3. 5.3+ won't need the libmysql.dll, so I'd suggest updating the PHP version anyway if you are < 5.3. If not, check the httpd logs, it should be triggering an error if you've uncommented the extensions in PHP but it still won't load.