My yamaha psr A300 doesnt work in midi...

just use the USB connection from the keyboard to connect it to the computer. MIDI will work over USB just fine.

You will need to send it in for repair is my assessment.

MIDI is the language used by electronic instruments to tell each other what to play. When you press a key on a musical keyboard this keyboard will send a MIDI message to a computer or a musical instrument connected. The message says something like: start playing this note at this volume. When you stop pressing the key a new message is sent which says: stop playing this note.
MIDI messages can be transmitted using MIDI cables or USB. Old instruments had MIDI ports (round with five connectors inside) so a computer had to have a MIDI interface. Nowadays it's almost all done with usb.
The important thing to remember is that a MIDI message (and a MIDI cable) are just an instruction to play, they don't contain sound. The sound you hear when you press a key comes from the computer or the instrument connected to the keyboard, not from the plastic key you have pressed.
There is a lot more to say, but I would suggest you check wikipedia first and then come back with more specific questions.

I have a PSR-E323 and to connect it with my pc using a MIDI connector should i install driver from http://download.yamaha.com/usb_midi/ ??? I tried to install that.. An error msg popped up... error 25001 no device is connected. connect a device and try again. If the "found new hardware" wizard appears push the cancel button...What should I do?? Help me pls...

You can connect it with USB connection to your computer. It should be plug and play, but some drivers might be required.
You can connect it also via MIDI interface: - you will need sound-card with MIDI input connector, or you can use special MIDI device like M-Audio UNO.

Hi,
sorry I don't know how to solve your problem, but maybe you can solve mine.. :) I also got the Yamaha PSR-E323, but mine doesn't work in Ableton (as a midi keyboard). How did you get yours to work/ how did you connected your keyboard to the computer?
I hope you can/want to help me! :)

Yes. You will need to have a MIDI interface on your OC and also select in the sounds control panel of windows that you are using the external synth. The MIDI will expect the PSR to respond to the "general MIDI" assignments.

Your manual is here, but you are right, you put the disc in the drive, and then plug the kb into the computer with the power off, then when you turn power on, it SHOULD start the found new hardware wizard. If it isnt, the midi/usb connection may be faulty
http://www2.yamaha.co.jp/manual/pdf/emi/english/port/psre423_en_om_c0.pdf

I know this sounds weird, but make sure you connect the MIDI-OUT cable to the MINI-IN port on the keyboard, and the MIDI-IN cable to the MIDI-OUT port. Backwards and illogical, but there it is.. I had the same problem; now it works flawlessly.

i have the same problem please someone help us.