What skin object are you using SolpartMenu.ascx or Nav.ascx?  For custom navigation providers I suggest using Nav.ascx.  In fact, I suggest moving towards this object in all cases, as the Solpart one will eventually become obsolete.  Specifying the ProviderName in either skin object should work (in your skin.xml or as an attribute if you are using the ascx files).

While this document is not a step-by-step guide, it does cover the navigation provider fairly well.  The WebControl download page also has a sample asp.net 2.0 menu navigation provider available with source.

The line of code you mentioned simply passes in the default providername if none is specified.  Here is the code from that method.

Protected Sub InitializeNavControl(ByVal objParent As Control, ByVal strDefaultProvider As String)

  If Len(Me.ProviderName) = 0 Then ProviderName = strDefaultProvider

By specifying a providername in your Skin you can override this.  The entries for the provder need to be in the web.config (for this is how the provider model works).  You will need to add your own entry to the web.config.  The name that you choose for this entry is the providername you will use in your xml or ascx file.

I strongly recommend looking at one of the 4 other providers to base your code off of.