halftones for sandcarving (sand blasting)

I am doing halftones of colored photos to use them in sandcarvings. I am having trouble with making my halftone into a negative. also I have to print in cmyk and I am using a canon mp620 printer and can not find the cmyk setting. any suggestions

Moderator notes: It sounds like "sandcarving" might be referred to as "sand blasting" in some regions.