I own this exact lens. There should be two cable releases. One interfaces with the shutter release on the Hasselblad camera body to the Imagon, and the other with the lens shutter. Through a really convoluted mechanism (works about 50% of the time), the Imagon will trip the mirror in the body when the shutter on the Imagon is released. In practice, its way easier to manually trip the mirror lockup on the camera body.
For the softest effects, the f/stop on the lens must be wide open. As you stop down the lens iris, the lens will become sharper and sharper, until you notice no soft focus effect. Use the the lens wide open, and the discs to control the effect. You should notice that the discs also turn so that some of the apertures through the disc are blocked. This controls the amount of effect you get from the lens and exposure.
In use, what I do is focus the lens, manually trip the mirror lockup on the camera and then expose with the shutter release on the lens. This takes some practice, and use on a tripod - but, the lens is far from hand holdable anyway. BTW - the price you paid is really a good deal.
When I got my lens, I paid $1450 USD (employee price!) in 1994. The lens was not being imported into the US and I got it through a friend who's a representative for HP Marketing. The president of HP hand carried the lens back from Germany on one of his trips to the Rodenstock factory. You have a very low production lens that is no longer available. They work great - have fun using it. Search on photo.net, Bob Solomon (VP of HP Marketing) posted extensive instructions about 4-5 years ago on exactly how to use the lens to best effect.