Yeah, if you have the plastic Makin's gun (with the disks that aren't flat and metal), you have the one that's intended for use with their very-soft air-dry clay. You need the other gun (and disks)!
There are various types and sizes of clay guns... IMO none are perfect, but depending on what you want to do with it and whether you have finger-hand-arm problems, among other factors such as size, there are different ones which could suit you better.
If you want to read about all the types, and lots more about how to use them, clean them, etc., check out this page:
http://www.glassattic.com/polymer/claygun.htm
(If I had unlimited funds, I'd probably still go with my small plunger type guns as long as I could use them with my wood bellows as a pusher... that's the fastest way I found and the easiest on my body. However, I'd still probably get an All Metal gun from Polymer Clay Express with the smaller barrel that can fit inside and loads of disks and play toys that can be bought for it. If I didn't use it much, and didn't have physical problems, I might go for the metal Makins' one... don't know, have to think about it more).
Diane B.