I can find no reference to TDM on my iMac DV SE and I have no idea where to find firmware version.
Please provide explicit instructions.
It will be helpful if you can tell us precisely which model iMac you've got; if you bought it new, telling us when you bought it will probably suffice if you can't do any better. (Telling us your iMac's color will also work as long as it isn't graphite.)
Edit: Your choices are Blueberry, Grape, Strawberry, Tangerine, Lime, and Graphite (1999 model) & Indigo, Ruby, Sage, Snow, and Graphite (2000 model).