Motard, my advice is to contact the circuit directly, either by telephone (if your Spanish is good, no English spoken) or email (they will reply in English) :
http://www.circuitcat.com/ingles/circuit/contacta.asp
From recollection, there was no prebooked parking possible but circuit staff may be able to tell you where best to park and which carpark to use (as there will be separate parking for bikes and cars).