Hello Brian,
You could start by selecting the default "Inbox" View, so you won't need an advanced filter for FolderName. In the list of filters, in the "Contexts" group, you'll find (none) as an option at the top of the list of Contexts. Just select that and you'll have a view of the contents of the Inbox showing only those items with no Context assigned.
With MLO filters, there are usually several ways to achieve what you want. So, another option if you don't want to start with the default "Inbox" view (and I suspect you didn't manage to start with that view when you set up your filter), you could start with the default, unfiltered hierarchy view, set up an advanced filter on "TopLevelFolderName" contains 'Inbox', then configure the "show hierarchy" to include child items but not include parent items.
Hope one of those options suits you.
Stéphane