dsPic33f : How to interface ECAN with CAN High/Low Signal
You need to connect CAN_TX and CAN_RX to a CAN transceiver such as an 
MCP2561.  The CAN transceiver will provide the CAN_HI and CAN_LOW signals.  For an example, look at how they implemented the dsPIC33EV 5V CAN-LIN STARTER KIT evaluation board.  The schematic for the evaluation board can be found in Appendix A of the dsPIC33EV 5V CAN-LIN
Starter Kit User’s Guide.