Hi Anton, This is the procedure... First you need to choose which of the payment gateway you want to use and once you configured it out, proceed to their developers page and get their pay now custom code.

Once you have the code open your evolution and proceed to shopping cart. Understand shopping cart click on order management then Payment type list and click on Add then type and under type tick pay now then payment type choose Custom code. Once you choose custom code you will se a space to paste youe html code and you are good to go. 

Should it not work contact the merchant and seek help from them.