Well, first of all, you can only use it nested within another function that
will take that range and return a single value (like SUM()) because you can't
return an array into a single cell in Excel. Assuming that is what you want
to do, nest the ABS() function and commit it by pressing CTRL+SHIFT+ENTER.
For example:
=SUM(ABS(A1:A50)) (entered with CTRL+SHIFT+ENTER) will sum the absolute
values of A1 to A50.