Update current users:

SQL Query
update ubbt_USER_PROFILE
set USER_NOTIFY_ON_PM = 'yes',
USER_ACCEPT_PM = 'yes'

The default for accepting private messages should already be yes, but just in case:

SQL Query
alter table ubbt_USER_PROFILE
change USER_NOTIFY_ON_PM varchar(3) not null default 'yes',
change USER_ACCEPT_PM varchar(3) not null default 'yes'