Yes, you can use your shop credit and a voucher at the same time. Only several vouchers cannot be used for the same order.
If you want to redeem your shop credit, you must first log in to your account. Then you can apply the credit in the shopping cart.
After that you can apply a voucher directly in the order overview. You will come to the order overview after you have clicked on "Checkout" in the shopping cart. There is a grey text "coupon code available?" directly above the green field "PURCHASE NOW". Click here and you will find the field for entering the voucher.