You should see SmoothStepper......... in the list, if its not there then the plugin is not installed correctly or you have too many plugins active from other devices and thus it is off the list.
To check if the plugin is installed go to your Mach3 folder then the plugin folder and you should see the plugin there, it will have a .dll extension, if its still a m3p extension either double lick it or manually change it to .dll.
If you have Mach installed to a location other than C:/Mach3 you may have to manually place the plugin in the folder.
Hood