What do you want to use the cash credit for?
It will be automatically used to pay for out of bundle services such as international calls/texts, calls to non geographic numbers, premium SMS messages, sending MMS messages, etc.
If you mean that you want to use it to buy an add-on or membership plan then the option to pay using your cash credit should appear at checkout if there's at least enough cash credit on your account to cover the full purchase.
E.g. if you have €7.33 cash credit on your account and want to use it to buy a €12.99 plan then you first need to add an additional €6 cash credit to give you €13.33 cash credit and then you should see the option to pay for the plan using your cash credit at checkout time.
If you need to contact 48 support about this then see the links in my signature below.
