I am trying to create a new texture for a dress and would like to know how I can apply a texture to a specific part of the dress eg. a section of the hem having created the transmaps. How do you actually assign a material to a specific polygon without using something like bodypaint.

you can use uvmapper and then paint the map in photoshop

Attached Link: http://www.uvmapper.com/

I have used UV Mapper, but it's the applying of the texture. I didn't want to 'paint' the texture but use real materials - a jpeg. How do I apply these?

Then you'll have to use Poser's Grouping tool and select the polygons you want another colour on, then assign a new material to those. Theoretically you can do that in UVMapper too, but this is one of the times where Poser's Grouping Tool is actually more handy.