Well, I'm not an expert in coding or sometimes the language that's used, but here's my take on it in my version of ninglish .. hehe. that's nina's english.
With DNN, it's a framework, not really a CMS or Portal product - it's a Web Application Framework which has a range of tools built in.
The concept of having other products *plugged into* DNN has been something that's been developed over time. An example of that is the FTB (FreeTextBox) Editor. It's the core installed editor that you in anything that uses the Text/HTML editor functionality. But people wanted more functionality with the editor so the concept of Providers was born.. and the FCKEditor and the Telerik/RadEditor and the CutesoftEditor are all other company, commercial or free, depending on what you use and able to be *plugged into* DNN and utilise their product in DNN.
From my understanding, taking the menu / navigation controls is another thing since we already do have elements of third party products, like Rad/ComponentArt/DevMansion - Third party menus that can be *plugged in* and not use the existing Solpart Menu. But here's the catch - the modules require the Solpart Menu functionality - so there is still a fair it of work in creating the Navigation controls - which is like a global approach to navigational issues. Even if you don't use solpart or if you use another product, in fact Solpart is still used to manage the modules - or we couldn't do anything with it.
I could be totally off track with this explanation, but the whole concept of DNN is that it's a Web Application Framework - so we use it for sites, portals, intranets, application solutions, and the future growth will see more use of other products integrate with the funcationality of DNN, and that is great news for us all.
Going back to using the SolpartMenu Designer, you need to be sure you have only the one for your version of DNN installed.
I hope this gives a little more insight on the working of DNN -such a great product and so much learn.
Nina Meiers