Works best if you do two fresh installs of each program, starting with phpbb,
then zencart, using same database for both.
On installing zencart, you will get an option to link to phpbb.
Installing zencart via Fantastico will not work, because last option not available
to you.
Also, do not add members to phpbb, until you have installed zencart.
HTH

Addendum: Have to say that linking 2 progs together not ideal, if one
crashes or is hacked, will possibly affect the other prog.