Nested queries are expensive and if you have some sort of a primary key that both tables 'know', you should use a JOIN. Also, why are you PICKing PICKSL twice? ( )

Code:
SELECT p.Picksl, p.Prod, p.Desc, p.Pack, p.Manuid, p.hiti, p.oh, t.Slot FROM
PICKSL P LEFT JOIN Alter A ON A.Prod = B.Prod 
LEFT JOIN Test T ON P.SomeID = T.SomeID
WHERE B.Prod IS NULL