I dont know if its just the way you've posted that info in your question, but, it appears as though your connectionStrings is part of/'under' startup - I dont think it should be - try moving it 'up' a level' or at least change this
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
to
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
</startup>
Also make sure you have added a reference to the Configuration Dll ie System.Configuration