The Plus Pack is for purchase only when you join the SkyMiles program, according to the literature in my offer. Limit 1 per member.
They may have other packs and other goodies they give to elite flyers, long-term members, etc., but this particular offer is for newbies only.
Purchase price is $60 for 4 one-way upgrades (with ambiguous restrictions) + 5000 SkyMiles.