Did you spot the setting up payment modules in OpenCart
article that was in the previous one I mentioned?
You would need to use a merchant account such as from Authorize.net or PayPal in order to accept credit cards via OpenCart. What is the existing merchant account that you currently are using that you are trying to link up to the OpenCart site? You might need to get in contact with them, and find out if there is a way to use their system with OpenCart.
You might also want to checkout the OpenCart payment gateway extensions page
to see if any of those additional extensions might be what you're looking for.