Installation of Dialog Instance - Error

am trying to install a Dialog Instance (DI) but during the SAPInst I am facing with an error!

First at all I made some things/requirements in this new server like:

- Install J2SDK;

- Set Swap space (20 Gb);

- Install SQL Server 2008 x64.

So, after that, I run SAPInst with the following option: "SAP ERP 6.0 Support Release 3 --> Software Life-Cycle Options --> Application Server --> MS SQL Server --> Dialog Instance"

The error is on the screen "MS SQL Server --> Database Connection Information" when I put in field "Database Instance" the value <hostname>\<instance> (in my case I write PTREPRD\PRD which are the hostname of CI server and the sap instance PRD), after I typing the button "Next" it shown a message error which say:

"Cannot connect to PTREPRD\PRD Error(s) returned: SQL Server does not exist or access denied. SOLUTION: Check that you specified the correct name for the database server, that the server is running and you are authorized to log on to it."

Can you help me how to resolve this problem!?