You must have an older board, or perhaps you're not in the USA, I can't tell. But, Promethean stopped providing serial cables/connections long ago, at least here in the states. USB is much more reliable. To deal with the distance issue via USB, you can purchase and run multiple extenders/boosters as they come in 3m lengths as well. Or you can purchase a Cat5/USB converter and enough Cat5 to cover the distance.
My company has installed 10's of thousands of Activboards and we use the USB/Cat5 converter to cover long distances. Most converts support lengths in excess of 25m.