The datasource is by default .\SQLEXPRESS (its the instance where databases are placed by default) or if u changed the name of the instance during installation of sql server so i advise you to do this :
connectionString="Data Source=.\\yourInstance(defaulT Data source is SQLEXPRESS);
Initial Catalog=databaseName;
User ID=theuser if u use it;
Password=thepassword if u use it;
integrated security=true(if u don t use user and pass; else change it false)"
Without to knowing your instance, I could help with this one. Hope it helped