It does work, sort of. I just finished a single player campaign with several MP3 mechs.
The only way to get the mechs in campaign is through the Free Market. Each new MP3 mech has an equivalent chassis in Mercs (which I assume it was modded off of). For instance, the Pitbull is based off the Mad Cat.
Here is a
link to a forum posting that lists all the known equivalent chassis.
Getting the mechs through Free Market is a 4 step process:
1) Identify the equivalent chassis of the mech you want to buy. For instance if you want a Pitbull, you are really looking for the Mad Cat.
2) Buy up ALL of the Mad Cats in the FM.
3) Sell one or more back to the FM.
4) Buy one or more back again. At this point (the second time you buy) you are actually buying Pitbulls, but the avatar will still show Mad Cat. However when you look for it in mechlab you will see you actually now have a Pitbull.
This is the only way to get the new mechs. The only way to get the new weapons is to buy one of the new mechs, then strip off whatever weps come with it.
The side effect of all this is that the mech descriptions in Free Market are all screwed up as you saw.