Just on principle, if you received it with that "expiry date" I say send/take it back. You probably will not get the results you want.
From the info on the Woodland Scenics web page, it looks like you've bought lightweight spackle compound. It's a very light, foamy texture kind of like dessert topping.
If you do go ahead with it, you might be able to soften it a bit with water, but usually once that stuff starts to dry out, there's not much that can be done.
Andrew