You need to know the program which created the .dat files - was it a backup program? That is the program which needs to be installed to open them.
Once you determine which program was used to compress the files, install it's 64 bit version to open or extract them. If the program isn't available in 64 bit use a 32 bit OS to do it.
If you compressed the drive itself, decompress it in the Properties box. How to read and open .DAT files in Windows