If you don't need delivery just go to the cargo area and you will find agents there. We were phoned when our freight arrived, we went to the airfreight place, there are guys in beige uniforms who take your documents and fill out the forms for you then give you your goods. we paid 200 riyals (would probably have been cheaper if we had haggled the price) there are also some customs fees which you pay by credit card at the counter (the agent will tell you and assist)

If you need delivery there are quite a few agents who will sort it all out, they're not cheap though.