For my 51st birthday, I took a weekend trip to Boston on a gravesite photo tour. Early in the morning I visited Cushing's grave to the south in Scituate, and later that afternoon I went to Mount Auburn Cemetery in Cambridge.
My god this one was hard to find.
Get you a map from the main office at the entrance. The grave is on a terraced path called Olive Path on the eastern slope of the hill atop which sits Washington Tower, a major structure at Mount Auburn Cemetery. The front of Curtis's completely obscured by shrubbery, making it nearly impossible to spot. His grave is plot number 532, and following the numbers on the slope will be helpful in finding it.