In any visualisation, AA can really only filter by whatever values have been tracked to its eVars, props and other dimensions.
So, first question is: are you tracking your page URLs to any eVar or prop?
On the other hand, maybe your pages are already being tracked to the Page dimension with human-readable names instead of URLs (depending on how your AA implementation has been done). If so, then you can use the Page dimension in your Flow visualisation instead.