Extract the c4d files from the .zip or .rar. Go to the containing folder, right click on the one you want to use and select Open With > Photoshop CS4. Then it should open it as a new document in Photoshop with a transparent background. Select All and copy it then paste it on a new layer in your sig. Then Transform > Scale it to fit the way you want.

When you are viewing your c4ds (or renders for that matter) deciding which one to use, always Open With, instead of copying the image, because you won't get a transparent background if you do.