Can not see through the lens to take picture
When looking through the DYNAX300si eyepiece every thing is dark
An SLR camera such as yours should provide an image in the eyepiece as long as the lens (1) cap is off and the mirror (2) is in the DOWN or 45 degree position.
The lens projects an image into the camera body where it shines on the mirror (2) - which reflects the image UP onto the focus screen (5). This allows you to compose and focus your shot thru the prism (7) and eyepiece (8). When satisfied, the releasing of the shutter causes the mirror (2) to slap up against the focus screen (5) (that's why the image disappears until the shutter closes) to allow the image to be projected straight back on the shutter (3), which opens and closes as determined by the shutter speed and exposes the film or sensor (4).
If you remove the lens, you should see the mirror at a 45 degree angle when looking straight into the camera body. The mirror should be reflecting the focus screen for you to see.
It is possible that the mirror has become dislodged or damaged (as can happen when lenses not designed for a certain line of camera bodies are introduced). It may or may not be readily apparent as to how to reposition the mirror - especially if internal parts are loose or missing.