You're not really going to see it from most angles. Partners Phase II and another lot yet to be figured out are going to block it from Grand Union Blvd, and the only remaining view is from the tracks. Or the Casino, heh.
IIRC, the garage isn't just for Partners either. This thing has a full retail base, in some sections two floors of it. Lots 7, 8 and 9 aren't provisioned for an above ground parking component, and lot 5A (the hotel+condos) is only going to have a tiny, single floor garage in the basement. Remember the existing mall is eventually getting redeveloped as well, alongside a separate, huge, non-Federal Reality redevelopment that's just getting started by the Home Depot.
The plan seems to be to have the majority of tourists park in the Partners garage, then walk through the big hole in the middle of the building that lines up with Assembly Row, through the pavilions between blocks 7 and 8, and into the rest of the mall.