You have left out what your skin object looks like.  Did you specify a ProviderName in it or not?  My guess is you didn't and that is your problem.  By not specifying one Nav.ascx will use the default name "SolpartMenuNavigationProvider".  If you were to specify one as "MyProvider" and use your first web.config entry it should work.

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Other than your connection string and your new provider entry you should not need to modify your web.config.  Let me suggest downloading the documentation and having a look at the DotNetNuke Skinning.doc file.  This will give you some insight as to how DotNetNuke is meant to be skinned.

If you are just interested in changing the default DNN-Blue skin for experimenting, the skin file is located in this folder

[DotNetNuke Folder]\Portals\_default\Skins\DNN-Blue\Horizontal Menu - Fixed Width.ascx

Open up the ascx file and find the following tag

<dnn:MENU runat="server" id="dnnMENU" />

Add a ProviderName="YourNameInTheConfigFile" to it

 

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