Are you following these steps:
1. Open your Mac's terminal and type:
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
brew install dfu-util coreutils gnu-getopt
2. Download the latest Yocto image from: IoT - Intel® Edison Board Download | Intel® Developer Zone and decompress it
3. In terminal, go to the place where the Yocto image was downloaded and into the Yocto image foder.
4. Type ./flashall.sh
I was not able to get flashall.sh to work properly, but I did was able to get flash edison using the Intel Flash Tool:
On OSX it's called Phone Flash Tool Lite, but it got the job done!