In the scope of Docker Machine v0.5.1 release, there were some backward & forward incompatible stuff merged. So, "parallels" driver v1.0.1 could not be used with Docker Machine 0.5.1+

$ docker-machine ls
WARNING >>> Error attempting heartbeat call to plugin server: rpc: can't find service RPCServerDriver.Heartbeat
Error attempting to invoke binary for plugin 'parallels': rpc: can't find service RPCServerDriver.GetVersion

Docker Machine issue: docker-archive-public/docker.machine#2325

Current status for Parallels driver v1.0.*

• It works fine with: Docker Machine v0.5.0 / Docker Toolbox v1.9.0
• It doesn't work with Docker Machine v0.5.1+ / Docker Toolbox v1.9.1+

Please, be aware while updating Docker Machine / Docker Toolbox