Is SQL Server instance a named instance or default instance? If it's a named instance, you will have to include that in the connection string. If it is a default instance, you should be able to enter the server name or the cluster name. Can you please provide more details so we can help you solve the problem?
Alan H
MCSE - Data Management and Analytics
Senior SQL Server DBA
Best way to ask a question: http://www.sqlservercentral.com/articles/Best+Practices/61537/