pgadmin 3

In pgadmin3 I'm tring to add auto_inc column like this

ALTER TABLE public.modems  ADD COLUMN id serial;

But get this error :

NOTICE:  ALTER TABLE will create implicit sequence "modems_id_seq" for "serial" column "modems.id"

ERROR:  adding columns with defaults is not implemented
HINT:  Add the column, then use ALTER TABLE SET DEFAULT.


How to add auto_inc fields... thanx

PS. Can I use OID column for primary/foreign functionality, or it
has some restriction ...i mean i may not use auto_inc field ?