How long do you bake it for?
Metal tins covered with polymer clay are baked according to the thickness of the clay on them --it's no different from baking a clay piece alone as long as the clay is on the outside of the tin.
Is there any special treatment you need to keep the tin stuck to the tin?
Raw clay** will stick easily to smooth metal and to other non-porous materials so nothing is really needed to hold it on through baking.
And if there is a "mechanical hold" of the clay on the tin (for example, continuous clay from top to side, or even just around the side), no further adhesive is necessary after baking and in use. If the clay doesn't have a mechanical hold though (e.g., it's just covering the top where it's flat), then it would need an adhesive underneath --liquid clay would be a good adhesive to use before baking, or an adhesive that works well with metal would be good if used after the clay is popped off and glued back on (a 2-part epoxy glue, superglue, or E-6000, etc.... other glues could work too but not provide quite as strong a bond).
There's loads of info and lessons on covering Altoid tins (and other metal items) on this page at my site if you want to check it out:http://www.glassattic.com/polymer/covering.htm
...click on Altoid Tins
, under the Metal
(that page also discusses all kinds of other materials and items that can be "covered' with polymer clay)
**that is, except for the new clay called Studio by Sculpey
... it won't stick well by itself due to its almost fabric-like texture (in that case, you'd want to apply a coat of permanent white glue, then let it dry at least a bit so you'd have a tacky surface that would hold onto that clay better)