JP,
Disclaimer: I don't work for DNN, nor do I claim any kind of expertise. I'm just another user / developer like you.
I'm sorry you're having so many problems. I can't imagine your frustration. No... check that. I CAN imagine your frustration. Having been there myself.
I'm going to assume that you did an upgrade from your current version to the 5.3.0 version, correct?
Based on the error message it looks like you're missing a whole parcel of DLLS that you'll need to install. The navigation providers are in a separate directory called "providers" under the bin folder. (Don't ask me why the upgrade didn't install them for you, I have no idea) The solution is to get the right DLLs in the right place, so the compiler can find them, when it needs them. If you've done the upgrade, the web.config should have the entries to the location of the DLLs. Check there first to make sure the web.config has the entries for the missing DLLs. If you need an example 5.3.0 web.config file, I can send you mine.
I'd recommend you get the source for 5.3.0 and look at the bin directory and copy any and all missing DLLs from the source download into your website.
Make sure you are using WinZip or some other unzipping program to get ALL the DLLs. Windows might be blocking unzipping DLLs.