Try using the ip address instead of a name, example SQL Server connection string:
Data Source=123.4.5.6\SQLEXPRESS,1433;Initial Catalog=MnStores;Integrated Security=SSPI; User ID=pc1;Password=admin;
In SQL Server Configuration Manager->SQL Server Network Configuration->Protocols, check that named pipes and tcp/ip are enabled.
Check if your firewall is blocking the port.