Once you are logged in to your account simply click on "Deposit" which is located next to "Balance" at the top right of the homepage. A popup will then appear and you can select your deposit method and enter the relevant details to finish making your deposit.
Alternatively you can make a deposit through the "Manage your Account" section by clicking on "Deposit Funds" and following the same steps as above.
Once you have entered your card details, they will be saved to your account so you won't have to bother entering all your details every time you make a deposit.