I have successfully tested the attached sketch on a chipKIT PRO MX7 using Arduino 1.6.8 and the PIC core release 1.1.0 (See http://chipkit.net/chipkit-core-is-released-for-use-in-arduino-ide/). I have modified the attached Blink sketch to test the mapping of the board LEDs and test the serial port.
Yes, you are using the correct serial port.
1. Make sure that you ave programmed the chipKIT PRO MX7 with the "chipKIT_Bootloader_MX7ck" using MPLAB X. The boot loader is available on the Digilent web site. I have also attached the hex file below. To program the bootloader with MPLAB X, you will need to attach the PC to the DEBUG USB port. After programming the bootloader, you will no longer need to be attached to the DEBUG USB port.
2. Make sure you have JP11 shorted and R159 removed (See pdf file below).
3. Before attempting to load a sketch, hold BTN1 down while pressing the reset button. If the chipKIT Pro MX7 is reset and ready to download a file, LED1 on the processor board will flash about 4 times per second.
4. This sketch sets the UART for 38.4KB - Make sure you set the serial monitor for 38400.