If you are talking about audio karaoke files, then the CTK-810 has no way of playing them. You will need to use a separate player/amplifier/speaker system (boom-box, etc), but if you are talking about MIDI-based karaoke files, then any MIDI file player or computer based DAW software program with a MIDI sequencer should play MIDI files through the CTK-810, using the CTK-810 as a sound module, but they will not "load" MIDI files into the CTK-810's User Song Memory. Neither will a computer's operating system file manager. To load files into the CTK-810's User memory, you will need to download and install the Casio SMF Converter software from this site:
http://music.casio.com/e/smf/
Do not install the SMF Converter software that came on the CD with the CTK-810, as it has problems that may prevent you from loading files into the keyboard's memory. If you have already installed it, uninstall it and download the updated converter software. The CTK-810 is not Class Compliant, so you will also need to download and install the appropriate drivers for your operating system from this site:
http://support.casio.com/en/support/download.php?cid=008&pid=20
If you have 64 bit Vista, the 64 bit Win-7/8/10 drivers will also work with 64 bit Vista.
The Casio SMF Converter software will not recognize karaoke files with a .kar file extension, but they are really just MIDI files with a lyrics track and a .kar extension. You should be able to get most or all of them to work by changing the extension from .kar to .mid with your computer before running them through the converter program. If some will not load, you may need to load them into a DAW program and delete the lyrics track, if its presence makes the file too large and uses too much memory. That is not a bad idea anyway, because the CTK/WK keyboards do not really have a lot of memory for storing song files. If you were expecting the CTK-810 to display the karaoke lyrics, it does not have that capability.