I did not play with wpmu 3.0 yet so I don't know what 'network activate' is.
in any case:
FireStats for wpmu is designed to createa SINGLE set of tables that
will be used by the entire wpmu site.
you should activate it using the administrator in the first time, which
will create those tables.
subsequent activations by users are not supposed to have their own set
(see the installation instructions at http://firestats.cc/wiki/WPMU