Looking at the title (not the text) of the original post gives a clue. "Trying to get null values" sounds like they want to retrieve rows with NULL dates as well as the MAX. Here's a stripped-down version:

CREATE TABLE po

(ID int not null, location int, poDate date)

INSERT po (ID, location, poDate)

VALUES (2, 24, '2014-07-08'),

(2, 24, null)

DECLARE @poID int = 2;

-- Existing logic

SELECT * FROM po WHERE id = @poID AND location = 24 AND

poDate = (SELECT MAX(ISNULL (poDate, '1900-01-01')) FROM po WHERE id = @poID)

-- Proposed logic

SELECT * FROM po WHERE id = @poID AND location = 24 AND

(poDate IS NULL OR poDate = ISNULL((SELECT MAX (poDate) FROM po WHERE id = @poID), '1900-01-01'))