the salt does not need to be encrypted. the point of it is that it makes a generic rainbow table useless, because the crackers need to compute hashes themselves for all passwords.

as they said, the purpose of hashing is to slow down the crackers, because they need to find the string that produces that hash. a rainbow table cancels that, it makes password lookup for an account almost instantaneous. but a rainbow table is only really useful for unsalted hashes, because for salted hashes a different rainbow table is needed that takes the salt into account.