My DXG-395v has trouble focusing in telephoto mode

It's not actually a telephoto lens, it's a telephoto adaptor. I have used something similar, though not this actual model, and only on MF lenses. Have you tried at all focal lengths of the zoom - it may only operate within certain limits. If it happens constantly, then you may need to switch to manual focus.

may be there is some problem with your shutter speed and lens

There are several reasons why you have to use this lens in manual mode. There is no chip in it and no focus motor. But even if there was a focus motor, your camera could not focus automatically, because the highest lens opening (smallest aperture number is f/1:8 on 1300 mm it is even f/1:16) Your camera only can focus on AF if the aperture is or is smaller than f/1:5,6. For the rest, you should chose the M on your mode dail. and then you can just press the shutter release button. Look on the screen how the histogram looks. If the histogram is to the left, the picture is to dark, so you should choose a longer exposure, if the histogram if far to the right, the picture is to light, so you should choose a shorter time. Have fun.

Did you try putting the camera and lens in Manual Mode and focusing by hand? On Many cameras there is a tiny switch on the lens that limits your focus ability to either FULL or LIMIT.

The lens is a manual focus lens. It doesn't matter which mode you have the camera in, you'll still have to focus it yourself.

Yes and no. If it doesn't come with one, you'll need a T-adapter for Nikon in order to mount the lens (you can get one from Vivitar, as well as from other manufacturers). The lens is not an autofocus lens, so obviously you will have to focus manually. It's also a mirror lens so it has a fixed aperture. It's what Nikon classifies as a non-CPU lens, so you cannot use the D90's exposure meter. You will have to shoot in Manual mode.

Lock it at f/32. Autofocus lenses have a lock for smallest aperture (highest number) because that is wht the camera body expects. The lock won't engage at any other position.

It works fine for me. Reember it works ONLY in P mode. The problem is due to the Fuji E550 does not provide focus indicator, you have to judge the image to see when the object is in right focus.

In wide-angle macro mode, the minimum focus range is as close as 30cm from CCD(21cm from the lens front.) At this point, the magnification is 0.03x (equivalent to 0.12x in 35mm format). (Focal length is7.2mm. At maximum magnification 283mmx213mm of the subject can be captured.) In telephoto macro mode, the minimum focus range is 25cm from CCD(13cm from the lens front )and the magnification is 0.177x (equivalent to 0.7x in 35mm format). (Focal length is42.7mm to 50.8mm. At maximum magnification 50mmx37mm of the subject can be captured.)

Using the Macro mode, it is possible to zoom through all ranges from telephoto to wide down to a minimum distance of 7.9 inches.