Associating your credit card to an account is a feature very important to your balance reports and predictions. To understand better the importance of this feature, check out the article Credit Card: What's the "Associated Account" feature good for?
Let's see how you can choose the account you wish to associate to your credit card.
You can choose the account related to a credit card when creating a new card. On the credit card menu, touch the '+' icon on the superior right tab in order to create a new credit card. You can also click the '...' (more options) on the credit card you wish to associate to an account. This will allow you to edit a credit card.
On the edit credit card menu, choose the option 'Attach to an account' and select the preferred account.
Done! All the expenses from this credit card will appear on the charts and reports of the chosen account. This option can be edited anytime you wish.