In the editor, go to the staticmesh browser, select your model.
In the property window at the bottom the top entry is material. If you have 3 materials (sorry, coming from Maya there, dont know how it works with XSI) on your mesh when exporting it from XSI you should see 3 different materials there. (e.g. there should be [0],[1],[2] when you extend the view)
After you imported the textures, put the appropriate textures in the according material slots. (Select the texture in the texture browser, swithc to the mesh browser, select the material slot you want this texture to go and hit the 'use' button, rinse and repeat for the other two)
When the UVmap seems to be an ass, i think i remember some issue where the texture has to be inverted to work correctly in UE. I think someone else might have to comment on that, but iirc i had that issue once.
Let us now how it works out. (the plugin looks a little different from the one maya uses)
worluk
btw. may I use this place to rant about that DAMN BUBBLE NULL TEXTURE?
I hate it, every time i see it my stomache turns in a very hurting manner