The following question came up in discussion, "Where is the Arduino supposed to plug into the chair?". 

The DB9 (Serial port) adapter in this design is intended to be plugged into a power wheelchair that utilizes the RNET interface. Both the Input/Output module and LCD Omni module can be used. Both the I/O and Omni module have documentation on how to program the interface to allow button press input.

The chain should look like below

Computer -> Arduino -> DB9 Connector -> Power chair with RNET interface

Hopefully, that clears it up!