DSLRs read the aperture data from the lens via electronic connections. Reading ƒ/00 basically means that either there is no aperture data on the lens' chip or that there is no electronic connection at all, which is probably the case with your DIY pinhole lens.

The diaphragm resides always in the lens, not in the DSLR. So: there is no diaphragm if you haven't built one — and it would be rather useless in a pinhole lens.

There is no setting to tell the camera that there really is a lens even if the camera doesn't recognize it. This is more a problem for the autofocus system, your DSLR should be able to take a picture even without a lens.

The reason your photographs come out almost black might be because your exposure time is not sufficient. To determine a "correct" exposure, you should either calculate the aperture of your pinhole lens or go and experiment, which is usually easier or at least a lot more pleasant option than to worry about the math.

Anyway, Wikipedia gives a concrete example on calculating the aperture of a pinhole lens:

The f-number of the camera may be calculated by dividing the distance from the pinhole to the imaging plane (the focal length) by the diameter of the pinhole. For example, a camera with a 0.02 inch (0.5 mm) diameter pinhole, and a 2 inch (50 mm) focal length would have an f-number of 2/0.02 (50/0.5), or 100 (f/100 in conventional notation).

If your pinhole's aperture is ƒ/100, note that it is approx. 5⅓ stops* smaller than a ƒ/16 lens, and therefore needs approx. 39 times** longer exposure, if other settings are intact. The Bulb setting you are using doesn't have a fixed exposure time — with bulb, the camera exposes as long as you hold the shutter.

Viewfinder can't be much helped, though. You could get a rough estimate of where you are pointing your camera at by first using a regular lens and when done switch the pinhole.

*) See: http://imaginatorium.org/stuff/stops.htm
**) (100 ÷ 16)2 = 39,0625