. . . . how do I install my program on two computers without telling me the Serbia number has been registered to another compuer . . . .
You can not, for MEP 2013 1 licence = 1 computer. installation
MEP 2015 has a new licencing regime. You can install it on more than one computer however you can only use it on one computer at a time and you must register the software to that computer. To use MEP 2015 on another computer you can change the registered computer however you are allowed to do this once every month - see Licence Verificationhere