Building on what Rafeal's response; you need to upgrade to SQL Server Express.
SQL Server Express LocalDB is a stripped-down version with a lot of limitations; one being that it does not support concurrent connections. See the link below - the part explaining the concurrent connection limitation can be found in the last sentence of the Permissions section.
http://technet.microsoft.com/en-us/library/hh510202.aspx
You can install SQL Server Express here:
And in Visual Studio you can use the Database Explorer to create a Data Connection to the new database to create your tables and any stored procedures you need.
If you are not familiar with .Net and SQL Server, you can connect to the SQL Server Express database easily by using the System.Data.SqlClient namespace. Since it's designed for SQL Server products, it is supposedly more efficient than using ADO.Net or other data providers. And, at least, I have found it to be quick and easy to use. For example, since the provider is explicit, you don't need to specify a provider in the connection string, resulting in a shorter string. For more information on the connection string, see this page:
And you can find C# and VB examples of opening a connection at the bottom of this page:
http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection.aspx