I was working through Todd Bleeker's book and ran into this error with the asp.net web site administration security tab.

The connection name 'LocalSqlServer' was not found in the applications configuration or the connection string is empty. (C:\Windows\Microsoft.NET\Framework\v2.0.50727\Config\machine.config line 141)

Basically, i had to go add the corrected database name into the machine config so that i could add users to the asp.net sql database.

Looked like this for me:

<add name="LocalSqlServer" connectionString="Data Source=localhost;Initial Catalog=SharePoint_Config;Integrated Security=True" providerName="System.Data.SqlClient"/>