When exporting textures from ZBrush to XSI make sure you flip the texture Vertically before you export it or it will be upside down when you bring it into XSI. this can be done, in the export menu of ZBrush I think theres a button called flip V or flip Vert, something along those lines.
As for the second part of your question, the texture editor isn’t really for applying the texture rather just editing where the vertices of your model are on your texture. To apply a texture select your model and open the render tree, which is view > Rendering/Texturing > Render Tree (or the shortcut is 7) then drag an image node onto the tree (you can search for it in the search bar) then choose your texture and make sure you set the texture projection that you used in ZBrush. Then plug it into the diffuse of your phong or whatever you are using.
Hope this helps