I just bought a tamron di 2 AF 55-200mm f/4-5.6 LD macro for my nikon D40 and i can't get it to autofocus

http://www.tamron-usa.com/assets/pdfs/272E.pdf
This is the manual for the Tamron SP AF 90mm
F/2.8 Di Macro 1:1 (Model 272E)

See if this site is of any help

http://camera.manualsonline.com/manuals/mfg/tamron/tamron_camera_lens_product_list.html

Excerpted from a www.photo.net review of the lens:

Though the Tamron AF 55-200mm Di II LD Macro has "Macro" in its name, it's not really a macro lens. Maximum magnification is about 1/3 life size (Tamron spec it at 1:3.5) as you can see from the following image below, which was shot at 200mm and at the closest focus distance (0.9m/37.4"). This isn't too bad, but it's more of a close focus telephoto lens than a "macro" lens (bolding for emphasis added).

Try cleaning the lens and camera contacts gently with a lint-free cloth

It will fit but no AF.

change your VR unit

When using a TC, as the perceived focal length increases and the aperture falls off, AF struggles to work except in very bright light.

There should be a switch just in front of the back of the lens marked "AF" and "M". Set it to M for manual focus, AF for autofocus.

Autofocus function on the D40 only supports lenses with the
AF-S feature, which have an autofocus motor built into the lens,
instead of using an autofocus motor drive built into the camera.

The Tamron lens you have, does not have a built in motor and
the autofocus function relies on the motor drive in the camera.

It will not work with the D40 or D60, but it will work with other
Nikon digital SLRs, such as the D80. Need to get an AF-S
type lens or upgrade to different Nikon digital SLR. Unless
you do this, you will have to manually focus the Tamron lens.

All of the Nikon DX (for digital) series lenses are also AF-S
type, so they will work with the D40, which is designed as
an entry level digital SLR, therefore it is intended primarily
for use with DX lenses, which are typically sold in a kit with
the D40. Most common one is Nikkor 18-55mm DX AF-S.

Other Nikon AF-S lenses made for film cameras (FX type)
will also work with the D40, but these tend to be expensive
professional models. DX series lenses tend to be more
affordable. You might consider the 18-200mm DX AF-S
as an alternative to the Tamron, but these are not cheap.