Yes, it needs to stay moist. Apparently it loses its electrical charge and is useless once dried out. I have also read that the shelf life of even properly stored resin is only like 1 year, so even if moist it may not work as well as you'd like. If you have a tds meter that shows RO only tds and then DI tds, you can tell when stuff needs to be replaced. If the bags you have now have mostly dried out, you could try using them and see what happens to the TDS, although I am unsure if dried resin will release stuff you don't want.
Another point is I have read that mixed bed DI works best when the cation and anion resins are thoroughly mixed. So if you are using them in a single DI cannister, make sure you mix everything really well, and then pack the resin in the cannister well so it doesn't separate back into anion / cation layers.
