Casio PX-320 Keyboard has no MIDI driver for MacOS

Like your Yamaha midi needs drivers.
https://www.solvusoft.com/en/update/drivers/music-keyboard/casio/privia/px-120/model-numbers/

Newer Casio models that have "class compliant" USB interfaces such as the PX-330 would work (plug and play) since they now have "class compliant" drivers.
Unfortunately PX-300 is an older model that would need specific drivers and Casio only made drivers for Windows PCs.

PX 410r uses IDES 4.0 downloadable from
http://www.casio-europe.com/euro/emi/specials/ides/downloads/
and not another version.

I suggest you uninstall the Casio USB driver from Control Panel'system'device manager then Reboot the PC without the USB cable.
Download latest USB driver from :-
http://support.casio.com/en/support/download.php?cid=008&pid=20
64 or 32 bits versions are both available.
Load driver first. Plug in the usb cable only when software tells you to do so.
Uninstall Data manager if wrong version.
Install IDES 4.0

Make sure you follow the steps of the procedure below exactly. Connecting incorrectly can make data
send and receive impossible.
1.
Turn off the digital piano and then start up
your computer.
• Do not start up the music software on your computer
yet

3.
Turn on the digital piano.
• If this is the first time yo
u are connecting the digital
piano to your computer, the driver software required
to send and receive data will be installed on your
computer automatically.
4.
Start up your computer's music software.
5.
Configure the music software settings to
select one of the following as the MIDI device.
CASIO USB-MIDI

Check the midi channel. It needs to sync to FL Studio. Channel settings you'll need to look up in the privia manual...can be laborious - but worth it in the end :)

use the audio output to send the keyboards internal patch sounds, or use a usb midi adapter to use the keyboard to trigger the garageband midi sounds, if the keyboard has a midi output, that is. the usb output requires software drivers to use, and IS NOT mac compatible. You may have to select a midi source in garage band preferences or settings if you are using midi.

WHich driver do you download for the midi? please, I dont have any idea. can you share a link where I can install or download the driver?

When you are using midi the sounds from your keyboard aren't sent anywhere. What happens is control messages are sent over the midi cable from midi out to midi in on the other device which must have midi sounds on it (in your case the sound card) to make any sounds. The messages can be things like note on/of or pitch bend etc. If you want to send messages to produce a result on your laptop and still here something you can loop the messages by running from midi out on the device end ( your laptop) back to midi in on your keyboard. A much better explanation is available here:http://www.indiana.edu/~emusic/etext/MIDI/chapter3_MIDI.shtml I also signed up to this site (its free) that has great people and lots of info:http://www.recordingreview.com/ I recommend downloading Killer Home Recording setting up which at this time is also free and has the best advice you could want in it. I'd also recommend checking out Reaper which is comparable to Reason, Cubase or Protools. If this helps please let me know.

Try downloading MidiOx and troubleshooting with it. It will show you if you have any midi activity and which channel your Casio is transmitting on etc.

Try this driver.
https://code.google.com/p/casiousbmididriver/