Kenko Extension Tubes NOT working at all on Nikon 70-300

A lot of those extension tubes do NOT work with the IF zoom lenses such as yours from Nikon. Not sure about your specifically, but because of the design, there are incompatibilities.

I'd return the tubes and buy a close-up filter set. Or... If you want to shoot close-up, buy a micro lens, it'll work far far better. A used 55mm or 105mm AI micro would be GREAT with that camera, and they can be found at a great price.

If it doesn't work the way you want, return it

What do you mean by "they don't focus." Are you trying to autofocus and the camera can never focus properly? If you are using an extension tube then I assume you are trying to focus on something very close. Your depth of field will be so narrow that you should manually focus. The best way to find out your new minimum focus is to set the focus ring to the min distance, then physically walk back and forth looking through the camera until something is in focus. You may only be a few inches away from your subject.

I don't have any experience with Kenko tubes or the 70-300, but my 55-200 has a MAXIMUM focusing distance of about 12 feet with a 36mm tube fitted. Minimum focusing distance is when something is actually touching the glass.

I have the NIKON 70-300 VR, and Kenko tube and use it on a D700. They work fine together. But, I find together they are not very practical because, as noted above, the depth of field is very thin, and the point of focus elusive. But, the auto focus will work. Setting the focus manually to "about" the right distance, then walking into the subject works just as well.

"buy a close-up filter set" I have both tubes and close-up filters and the tubes offer significantly better IQ (IMO) over close-up filters.

Just a simple question Andrea,
What do you expect those tubes to do for you that they do not ?
There is sometimes some confusion over them because they are called "Teleplus" which for some people seems to imply that they increase the focal length of your lenses, which the do not... ( for that yuo need a teleconverter ).
If you want close-up ( taht's what they are for) then the 85mm 3.5 or some other "Micro" lens ( Nikons terminology for Makro...) might do the trick.
In my experience extension tubes generally work better for focal lengths ranging from around 50 mm up to 135 mm, shorter lengths make your subject end up "inside "your lens" , greater focal lengths give trouble focussing because of the paperthin dept of field.. Zoom lenses are somewher in between, but you have to switch off VR because it can start "Vibrating" ( trying to stabalize without success...) sometimes.

"but they don't focus."

What an extension ring does is move the lens' focus ranges. Stock by itself, the 70-300VR focus range is 1.5 meter to infinity. When you add an extension, that changes the focus range closer hence allow you take closer shot. What you give up is that, the lens no longer can focus to distant objects. There are no free lunch.

You will need to get use to how the extension ring changes the focus range of the 70-300. Try this. Start out by standing about 1.5 meters from the subjects with no extension added. Set your lens to manual focus and to 300mm. Find a subject and manually focus to it. Take a small step forward and put on the thinnest 12.5mm extension. Leave the lens to 300mm and try focus it again manually. It should focus. From that move the zoom back and fore slowly and see how it effect focus and camera to subject distant. You will notices as you shorten the zoom, the focus range also change closer and vis versa. When you change to a longer extension the whole set of focus ranges get closer still. Have fun.

The 70-300 and 16-85 are variable-aperture lenses, and not particularly fast, especially the 70-300. Extension tubes have a light loss factor, and it's possible that between the slow aperture and the extension tube's light loss that you've exceeded the camera's ability to AF, especially if light you focus with is marginal (flash doesn't count for auto-focusing) or you're using a filter on the lens. With macro, an AF-assist beam may be out of the lens' field of view or be blocked by the lens itself. Same with the 16-85 on the long end, which is probably where you want to bee for shooting macro (pun intended).

The camera's behavior may be perfectly normal. If you're using the 70-300 racked out to 300mm (f/5.6 wide-open), you might be losing a LOT of light, and could be asking the camera to AF at an effective wide-open aperture of up to f/16, which probably won't work.

From Kenko's website:

"Please Note;

1. When using extension tubes the lens will not focus to infinity. The focus range will be greatly limited to a very close focusing distance.

2. There is light fall off when using any extension tube, sometimes the equivalent of 3 f-stops of light is lost when using multiple extension tubes together. This light lost can affect the camera's ability to auto focus. Manual focusing is recommended should the lens begin to "hunt" (not lock onto the subject)."