an example of multiply mapping is grassed areas.
say you have a regular 512x512 grass diffuse map, to get some decent detail level from it you tile it but it's easy to see the pattern. you can use a multi map, which will generally (i think) have fairly unsaturated and neutral colours for grass, to break up the tiling of the diffuse map. the idea is to scale the multi map so it is bigger than the diffuse.
so if you have 512x512 diffuse and 512x512 multi, you stretch the multi out to be 2x, 3x, 5x etc bigger and it serves to slightly change the colour of different areas of grass. it's quite similar to the background image blend principle that is featured in btb, where you might use a google earth map.
add is the opposite. you scale it smaller to provide more detail to the diffuese map. when you have t1+add+multi you can get some nice results.
t1=basic map, add=detail, multi=variation
you don't need any nvidia plugins for these, they are just regular maps, assigned to different shader slots. browse through the ISI mas files to get an idea (mills, toban etc). the file names are suffixed with "_add.dds" and "_multa.dds" or something similar, though it is only for ease of identification, not critical to their functioning.