IsNumeric is the VB function for testing if a value is a number or not...
ISNUMBER is the worksheet function equivalent of that. Also, you don't
really need some of those parentheses as what they surround are
self-contained. In addition, you do not want to use the double unary (minus
signs) in front of the cell reference (if the cell contains text, it will
produce an error... plus, that is why you are using the ISNUMBER function).
This is the formula you should have tried...
=SUMPRODUCT(--ISNUMBER($D$10:$D$500),$D$10:$D$500)
HOWEVER, you don't need to have your formula do all that work... SUM will
skip over text and only add numbers. So, you can use this formula instead
and it should do what you want...
=SUM($D$10:$D$500)