Well first you must know from which coins are those .dat files. If you know for sure then download their wallets and open it from the official wallet of the coin.
It's most secure to do that from the official wallets only.
Of course you can do that in the top right menu FILE -> OPEN and find the .dat file where you have saved it and open it.

But again, you must know from which coin is that .dat file or you won't be able to open it
cheers*