Content pipeline importers and processors run inside Visual Studio as part of building your game. This means you can't just debug into them like you would for normal game code.
Fortunately, the CLR
provides a handy way of hooking a debugger up to any managed program
you like. You just need to modify your processor code, adding this line
at the point where you want to start debugging:
System.Diagnostics.Debugger.Launch();
Now when you build your game, when the processor hits this line it
will pop up a window titled "Visual Studio Just-In-Time Debugger",
listing all the possible debuggers that are installed on your system.
You will generally want to pick "New instance of Microsoft CLR Debugger
2005", which will connect to your processor and let you debug what is
going on during your content build.