Updating Sparkfun RGBLED matrix backpack

I purchased one of these a few weeks ago: [1] http://www.sparkfun.com/products/760 I've been trying to update the backpack for it to version5 through avrdude, but I keep getting this error after it confirms the baud rate:

 avrdude: stk500_recv(): programmer is not responding

The serial port is correct and so is the board select (Mega2560) This is what I've been entering in terminal (yes, Mac):

 avrdude -P /dev/cu.usbmodemfd121 -b 19200 -c avrisp -p m2560 -F -v -e -U flash:w:RGB_Backpack_v5.hex

Can someone please help me?

Do you use an ISP programmer? You cannot update the firmware with just an USB2Serial cable/adapter. An Arduino has a special bootloader to exactly allow this (USB2Serial) but for updating that bootloader you have to have an ISP programmer too. For that update a special serial interface (SPI) is used, not the standard UART.