Use the Custom gateway setup wizard which can be found at the bottom of the Gateway selection page in Admin >> Cart setup. The only data you are asked for is your Merchant ID at Beanstream.
Once the initial configuration is done you will probably want to improve the Thank you/Decline pages. You can do that by entering HTML in the boxes at the bottom of the Custom Gateway setup page. There is no real limit on the amount of text you can enter. Don't try to load images remotely into these pages unless they are on a secure server.
There are couple of changes you should make in your Admin area at Beanstream.com.
On the Payment form customization page select the options as indicated below. You can customize the rest of the form as you like. By default it is very basic but it works.
In Configuration >> Account settings >> Order settings enter the following URL for both Transaction Response pages:
where # corresponds to the server which your cart account is setup on.
URL to this page: https://www.mals-e.com/tpv.php?tp=11