If you have been offered (or paid for) LineLeap credit, this value will be visible in your account. To use this credit at checkout, select "Add a new payment method" and select the "LineLeap Credit" option from the list of choices.
Please note that payment cannot be split, so you will need to have the full value for your pass in your LineLeap credit. If the pass costs more than you have in LineLeap credits, you can add credits to your account to ensure you can successfully process payment. On the account tab and under payment methods, you can add additional LineLeap credits to your account by hitting the "+Add" button.