Installation of the PL2303 and CH340 / CH341 / HL340 drivers is easy. Simple open the OS X installer and follow the steps.
Download the driver for your OS X version from the download dashboard:
Default is most times the right choice.
When the driver has been installed, you can check if it has been loaded. To check, connect the serial device, open Terminal and type one of the following commands:
For PL2303 device:
ls /dev/ | grep 2303
For CH341 device :
ls /dev/ | grep ch341
The result should show the accompanying cu and tty handle.