Ah - I see the problem. I misinterpreted:
>> However, when I start bro I get error: can't open drop.
to mean it was coming from the shell. But it's coming from Bro. There
isn't any policy script called drop.bro, so it's telling you that "@load
drop" failed.
You turn on address dropping by
@load scan
redef can_drop_connectivity = T;
and redefining "drop_connectivity_script" (which it sounds like you've
already done).
Vern