I am having a very similar problem with a SainSmart Arduino Mega 2560. However, the arduino loader is not able to find the /dev/tty.usbmodem1d11 .
Are you able to find this port when you plug in the Arduino with the usb cable?
When I use an Arduino Mega from "Arduino" the port is correctly found and it works perfectly.
I have seen a thread saying that the usb cable can be finicky but I haven't been able to tweak the cable to get a different result....