how to open cash register?

I'm not sure what you mean by 'open'. if you mean just open the drawer, use the no sale button. If you mean it is showing a CLOSED error, you need to log on with your id code and the clerk button. If you need a manual, one of the users has it here:

http://the-checkout-tech.com/manuals/cashregisters/Samsung_ER-4615_operating_programming_manual.html

Registerman
October 2013