Creating a Linux boot drive on a MAC for a Kangaroo Mobile pC install
In your dd command,  

sudo dd if=~/Desktop/linuxmint.iso of=/dev/sdx bs=1048576


/dev/sdx for the output is incorrect.   


This is a Linux convention, not macOS; use /dev/diskX.  
X is the specific disk identifier.  If you are using instructions from a Linux site, they are most likely providing the instructions for doing this on Linux (not macOS) using X as a variable for the specific identifier since the identifier can be different for each user and/or instance as the device is removable.


In Linux it's /dev/sda, /dev/sdb, etc.  In macOS/BSD is /dev/disk1, /dev/disk2, etc.

Also (thanks to user3439894 via comments) you need to make sure you disk is unmonted but not ejected.  Do this by issuing the command diskutil unmountDisk /dev/diskX where X is the number of your disk identifier.