It seems like sitetree only supports named url patterns or "exact" urls. Because flatpages doesn't have a named url pattern by default, you could create one, like:
urlpatterns = patterns(
'django.contrib.flatpages.views',
url(r'^(?P<url>.*)$', 'flatpage', name='flatpages-page'),
)
Include that at top level or put it directly in your root urlconf (at the end). And then put flatpages-page flatpage.url
into your tree item, check 'URL as Pattern' and it should work.