[ALUG] Using k3b to write an ISO image

I have just been trying to use k3B to write an ISO image and I really, really can't understand how to do it. E.g. I click on "Tools - Burn DVD ISO Image" and it immediately starts writing, I'm not given the chance to specify what ISO I want to write or anything like that. How do I simply tell it to burn an ISO image of a file I specify? -- Chris Green