You've probably got the wrong serial port selected. There are no other reports of issues and everything's working fine here.
I am using MP for the first time. I am getting the same issue as above. The serial port is correct too. :/
What ArduPilot firmware and version are you using?
The CLI is no longer functional in the latest ArduCopter firmware.
How exactly can I check that? Could you let me know, if not an issue? I checked out this post http://diydrones.com/forum/topics/how-to-check-apm-plane-firmware-v... but the link you mention there is dead.
I am trying to solve this issue here (below) and I thought the CLI in the MP could help me out. But I guess not. http://diydrones.com/forum/topics/how-to-output-pwm-values-from-uar...