Hopefully you've figured out how to texture in XSI by now since this topic is a month old, but it's not good to see an unanswered question. Here's how I work with textures in Mod Tool:
Go to Render> (Get)Texture> Image
. A Material window will open. Import an image by clicking New> New From File...
. Under Texture_Projection, click New> Unique UVs (polymesh)
. You've created your UVs. Now press Alt+7 to bring up the UV editor.
I hope that was helpful, but maybe you already knew this and were trying to use Ultimapper specifically? I am new to Mod Tool and modeling in general, and can't say anything about Ultimapper. But there's a tutorial
on texturing in Mod Tool at the SDK Docs where I learned this from.