There is a shuttle bus service between the Harderwijk train station and Walibi Holland. Check this link for more information and the timetable.
From Amsterdam/Almere: leave the A6 at exit Lelystad and follow the Walibi signs.
From Utrecht/Amersfoort: leave the A28 at exit Harderwijk/Lelystad and follow the Walibi signs.
Kiss and Ride
Are you dropping someone off or picking someone up at Walibi? Drive to the park and follow the 'Halen & brengen' (Kiss& Ride) signs.