Victor,
I depends how the file is laid out.  As far as I know .dat can be used by
many types of program, and the resultant file will be different.
It may have commas or spaces between the text /data you want to put in to
different columns
You could open it with notepad to see what it looks like.

In excel File, open, in the type of file at the bottom select 'all files
*.*'  find your file and click on it.
Depending on if your data is separated by characters (chose delimited) or
spaces (choose fixed width), click next
Select the delimiter and see the data preview, click next.
On the next step you can select the format of the data.
I think on one of the steps you can drag lines between the columns to change
them and double click on a line to delete it or double click to put one in.
I have just created a tiny example and it does not seem to do this.
You can go back to the previous steps to change a selection.

University of Illinois has a pdf on this which explains it all much better
than I can (and there are pictures)
http://www.dmi.uiuc.edu/excelimport.pdf

If you are still having problems post back with the structure of your dat
file, and the version of excel you are using.

Good luck, Barbara