There are many possible formats. Are the values separated by e.g commas or TABs? Is there a header?
Paste a few lines of your *.dat file here (if it is humanly readable) or attach it and we will be able to help better.
In the meantime, have a look at "Read from spreadsheet file.vi" in the File I/O Palette. It might just do what you need. A 2D array output will do just fine. Converting the data into a cluster of arrays will make it less convenient to handle in the code.