When I go after one of these very far away planets, I print out the view presented in Starry Night with Telrad circles and EP FOVs centered over the target. You need to see that star field to identify the location.
Neptune should look like a bluish tiny disk and not "bright" like a star. I would amp up the power a bit to get a good view. The 24mm should get you to the star field, but something stronger should get you the view for the planet, somewhere between 8mm and 14mm should do. If seeing is good, go stronger! Once you find it, the color will really stand out.