Have you checked to see if the named instance is listening on TCP/IP ? By default, I dont think SQL Express is "connectable" over the network if thats what you are trying to do. I am pretty sure, however, that you can enable TCP/IP and connect over the network. Check SQL config manager and ensure that its listening on TCP/IP, disable shared memory and named pipes, and then try connecting locally using Servername\Instancename first to ensure its listening on TCP/IP. Once you are able to confirm this, then try connecting from the app. Keep in mind also that if you're connecting using TCP/IP and SQL browser service is not running, you'll need to specify port number in your connection string.