There are many many causes of dizziness unfortunately, as it can sometimes be hard to find the exact cause....but I can assure you that the receptors in your knees are not one of them! There are specialists out there that are experts in causes of dizziness, try googling for one in your area. Sometimes the blood flow to the head can be compromised and cause similar symptoms, so I would get this checked out.
Re. unstable knees...this basically refers to the combination of ligament, muscular and neurological control in the knee, ie simply how stable it is functionally. There are many exercises you can be taught for knee strengthening, core strengthening, balance re-education etc that can help with this, so see a physiotherapist who can assess you for deficits and then teach appropriate exercises. Good luck.
Andy Schmidt
The Active Physio's Blog - Ideas and answers to common musculoskeletal health questions, from a physiotherapy/physical therapy perspective