Your version of Max 5 will last for 30 days, to continue using Max, you will need to reformat your hard drive & re-install.
The plugins needed are in the CD's of the game. It includes instructions as to how to make a basic map.
In the 1st month you will learn how to make maps, but it will take more then a month to make a full on map that is playable in GR, it is a lot of work, but man, it is worth it!
Yor best bet is to find a full version of 3Ds Max 5 or 4.
As long as you have Max 5 or4 and GR, you will be able to make maps/levels for GR.
Here is a great guide for making your 1st map:
http://www.ghostrecon.net/html/map-guide.htm
For learning how to use 3Ds Max, go to:
www.3dbuzz.com, they have video tutorials that are free and will teach you how to use Max,
also visit:
http://www.3d-palace.com/
They also have a great number of videos that are free, from basics to advanced.