To install the required driver on your Apple computer, you can download the driver here.
Opening the disk image (.dmg) file mounts a disk image onto your Mac. If you are running Mac OS X version 10.3, open the file FTDIUSBSerialDriver_10_3.dmg. If you are running version 10.4 or higher, you should open the other disk image, named FTDIUSBSerialDriver_10_4_10_5_10_6_10_7.dmg. The driver installer will open and guide you through the process.
Once the installer finishes and tells you that the installation was successful, you can open up Repetier Host and connect normally. If you are still having trouble with connecting, please refer to the How To Connect tutorial.