SQL Error on PostgreSQL!! Please Help

[admin]

I used the following ALTER table command:
ALTER TABLE test
ADD constraint test_key
PRIMARY KEY (id);

and this is the error i get:

Error - /opt/pgadmin/phpPgAdmin/sql.php -- Line: 107
PostgreSQL said: ERROR: ALTER TABLE / ADD CONSTRAINT is not implemented for that constraint type.
Your query:
ALTER TABLE test
ADD constraint test_key
PRIMARY KEY (id)

Please help !!!