As with most mods, activate them and start a new SP game. You can play some on-line, but you will need to find a server with the same mod.

sorry FD, but I have to correct:
Tygrys as the main author of Massive Addon (which is in fact a two years old combination of different mods from that time) insisted several times that the MA is NOT meant for SP-campain and MP but for Single PLayers who have completed the campaign, know every corner of the FL-Universe and search for more exploration and fun. (See News from 26/06/2003)
@bratch:
Watch out: FL-MA is very unbalanced as it is put together from other mods. There are also quite some bugs. (Nothing bad about the MA; but in the two years since June 2003 FL-modding has made big steps ahead).
If you want to enlarge your FL-experience NOW you better try one of the major mods as Evolution, TNG (Next Generation), Tool Kit (1.2), Monkey Universe, Free Worlds, Crossfire and so on. They all add spice and more to the original FL, some with more challenging fights, some with more exploring/trading.

EDIT: Date of News

Edited by - zazie on 9/7/2005 6:48:45 AM

Update, When I refer to starting a new SP game, I usually play Open Sp and not story line with mods. Its just more fun to enjoy the mod with out dealing with a story.