Hi Juliano,
That is the first script where this table is created so I don't see how it can say it already exists unless you somehow ran that script previously. Only way I could see that happening is you were working from my svn sandbox. When I tested pgsql upgrade no error occurred.
I recommend if the tables mp_lettersubscribe and mp_lettersubscribehx already exist then drop them and visit setup again.
Hope it helps,
Joe