Not sure but you might want to try this
=DLookUp("[FilterDescription]","tblFilterDescrip","[DescripIDNumber]= & Forms!frmProductSelector!cboProdDesc")
Again I am not sure but it would not hurt to try.
Just a thought, is [DescripIDNumber] defined as a number? If not you'll need to add single quotes. Also check that the bound column of cboProdDesc is a number too.