First, make sure you have the MicroSoft patch, PR-1 from their official site:
http://www.microsoft.com/games/mw4mercs/Then to the MekTek site;
Go to the downloads area..to Mechwarrior 4 Mercenaries...then to Merc Expansion packs...then download this one:
MekPak PR1 to MP2.1a - English
Description: Updates from Microsoft's PR1 patch to MekTek's MP2.1a. English versions only.
Version: 1 Filesize: 1 bytes
Added on: 06-Nov-2004 Downloads: 41352 Rating: 7.4 (7 Votes)
HomePage | Rate Resource | Details | Editorial
Then go back to the main page of MekTek, there you'll see a link to download the MP 3.
I also got these Maps from the "Maps" aera: Maps - MS Official (11)
Maps - Mercs - MekTek ( 8 )
Maps - Mercs - Converted Veng/BK (19)
From the New BattleFields site, I just got the Full installer, which has a bunch of maps and missions lumped together.
The Co-Op missions are excellent!
And We're using the Dynavers.net teamspeak server (teamspeak.dynaverse.net, pw is sfcrulz), you'll see the MechWarrior room.