I've finally had success with my robot. I can now control the wheel chair from my computer.
I've built a Java application which I call a virtual joystick.
The next picture shows the setup I use to control the chair. On the right hand side is the joystick controller for the wheelchair. On the left is the Arduino 2560 with a Freetronics 433mhz reciever board and a circut with two digital to analog converters. This circut is connected via a ribbon cable to the wheelchair controller. In the bottom of the picture are two wires that connect to the power supply of the joystick, this is to trick the electronics of the wheelchair into thinking everything is connected correctly.