Use Ctrl-Shift-Enter to enter the formula.
If you do it right, then Excel will show the formula in the formula bar as
{=AVERAGE(ABS(A1:A10))}
to indicate that it has been interpreted as an array formula. Note that
you do not type the curly brackets; Excel adds them automatically when
you have correctly communicated (by array entering) to Excel that it is
an array formula.
Jerry