The method can be used for all printer driver not only for the FS-1020MFP.

Please proceed as follows:
1). uncompress the file packages.zip
2). install first FS1020MFPGDI.pkg - sometimes the installer will prompt you to enter Administrator credentials
3). install second RastertoKPSLFilter v1.0.pkg - sometimes the installer will prompt you to enter Administrator credentials
4). before installing the third package, open a finder window and locate the path Library/Printers/Kyocera/kpsl/ and copy the file rastertokpsl.app e.g. to the desktop. Please obey that the before mentioned path and the desktop are on the same drive. Therefore copy (hold ALT button) is necessary.
5). install third AdjustmentPDE v2.2.pkg - sometimes the installer will prompt you to enter Administrator credentials
6). copy the file rastertokpsl.app which was backed before back to the folder Library/Printers/Kyocera/kpsl/
7). now you can add the printer FS-1125MFP via the standard printer & scanners setting