I'd love to help you, but I don't understand your question. I assume you mean the K750i phone, but ResEdit was used for manipulating the resources in Mac OS pre X. Please explain exactly what you want to do.
I know that I need to create the images in an image editor, but I've no idea how to put them together to form a theme. I remember using Res Edit years ago to take apart the help application and put my own stuff in it...that's the kind of thing I want...
Wooo! I didn't realise they'd released a Mac version of it!
There's also Mylasyk which had a java-based theme creator on it which was quite usable. Drag and drop the images/textures you want and it automatically resized them to the appropriate pixel size for your phone.