There isn't much more information right now than this, but the Des Moines Register reports Z will be with the Iowa Cubs starting Tuesday:
Carlos Zambrano will start his minor league rehab in Des Moines, Iowa Cubs officials said Saturday.
He is expected to be with the team during Tuesday’s doubleheader against Oklahoma City that starts at 5 p.m. at Principal Park. Whether he pitches Tuesday, or during Wednesday’s 7 p.m. game, also at Principal Park, is uncertain.
One more web link, from KCCI-TV in Des Moines, has the same information.
Neither article says whether he'll start or not, although that's a logical assumption. Nor does either article say if he'll make more than one appearance.