Can you confirm you have a real Mojo? If you recently bought it (in the last threeish years) we didn’t make it. Some of these work, some of them don’t.
Try using Mojo Loader, it should work.
Alchitry-Labs under windows 10 in my case fails to detect the com port correctly.
Only in some cases removing and replacing the serial drivers did it work but if I remove the usb connector and put it back, the com port is no longer detected.
A manual port setting system like in Mojo-Loader or IDE would be better.
My Mojo is not original.
No problem under linux. 
I just pushed 1.2.7 which should fix this.