Bus routes/timetables:- http://www.ctsa-portillo.com/horarios.html

I'd either contact the Captain's Rest (see link) and find out if they are running a bus to the Volvo Masters, or head out there on one of the packages organised by the Costa del Sol News.
http://www.captainsrestgolf.co.uk/

There aren't any buses organised this year, either by Costa del Sol Golf news or Viajes Celtic, so you will need to get a Portillo bus which has La Linea or Algericas on the front (not sure if these stop at the bus stop at 24hr sq in Benalmadena) - link supplied in the previous post should help. The bus will take you to Sotogrande, from there it's a 10 -15 min walk inland to Valderrama course.
Hope this helps
Liz