What you have posted is correct assuming that the extension_dir is correct. It sounds like that the PDO extension is not compiled on your server. PDO needs to be enabled in easyapache and it sounds like your reseller account does not have access to easyapache. Because of this you will need to get the server administrator to make the following changes in easyapache.
Goto WHM »Software »EasyApache (Apache Update)
Select "Previously Saved Config (** DEFAULT **)" and click "Start customizing based on profile"
On the "Apache Version" page click "Next"
On the "PHP Version" page click "Next"
On the "Short Options List" page click"Exhaustive Options List"
On the "Exhaustive Options List" ensure that PDO, PDO sqlite and PDO MySQL are selected.
Then Click Build on the bottom of the page.
Once easyapache completes you should have PDO :)