SQL Server Express connection string

Question

<configuration>

<connectionStrings>

<add name="ConnectionString1" connectionString="Data Source=(.Lens\SQLEXPRESS)\DatabindusingWPF.sdf; Initial Catalog=COMASTER; Persist Security Info=False; "/>

</connectionStrings>

</configuration>

I'm pretty new to this so bear with me..

 

I'm trying to create a connection string in an app.config

It doesn't work.  I get an error server not found or accessible. 

I'm trying to connect to my database on my local with Sql Express