the error suggests that the table does not exist in the database.
use the following SQL to create it in phpMyQdmin:
CREATE TABLE `pm1_mailing_links` (
`LinkID` int(11) NOT NULL AUTO_INCREMENT,
`MailingID` int(11) NOT NULL,
`LinkURL` varchar(2048) NOT NULL,
PRIMARY KEY (`LinkID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
2) is you set each of the packages up using the same admin username and password, logging into one should log you into the other.
3) adding columns to the pm1_users will not have an effect on the Power Messenger functionality.