You should first verify that the PSR-270 is sending and receiving MIDI.
If you don't already have the Service Manual, you should download it from ElektroTanya:
http://elektrotanya.com/?q=showresult&what=yamaha psr 270&kategoria=&kat2=all
Don't click any "Download" buttons on the download page, because they're ads and won't download the manual. Instead, wait patiently-- for maybe a minute-- until the first line of text underneath the box changes from "...processing..." to "Get Manual." The words "Get Manual" are the download link for the manual.
Pages 17 and 18 of the Service Manual describe the PSR-270's built-in test program. Turn the keyboard off and connect a MIDI cable from its MIDI Out port to its MIDI In port so it can send MIDI messages to itself. Then hold down a C# Major chord in the second octave (C#2, F2, G#2) while turning on the keyboard to start the test program. Run test 37, "MIDI Chk," to verify that the keyboard can send and receive MIDI.
If the "MIDI Chk" test fails, you may want to verify that "Keyboard Out" (function F82) is turned on-- although it should be on by default when you power up the keyboard. If it looks like the keyboard can't send and receive MIDI, get it checked out and repaired, or else replace it.
If the keyboard passes the "MIDI Chk," test, you should next verify that Windows can tell that it's connected. Turn off the keyboard, connect it to your computer using the MIDI-to-USB cable, and turn on the keyboard.
In Windows, go to "Control Panel," "Hardware and Sound," "Devices and Printers," and see if the device is shown. (I don't know what it will be called.) If you don't see it, right-click on the window's background, select "Device Manager," and see if the device is listed anywhere. (My Axiom 61 keyboard controller shows up under both "Software devices" and "Sound, video and game controllers.")
If Windows can't tell that a MIDI device is connected, you might need to install a driver for the MIDI-to-USB cable-- but I don't think it's supposed to need one, and if it does then the manufacturer should have included drivers with the cable or else should have drivers on their web site. If no such drivers were included and the company has no web site (which would not be a good sign), then the cable is most likely bad and should be replaced with a new MIDI-to-USB cable from a more well-known company that has a web site.
The rest of this post assumes that the keyboard's MIDI is working, the MIDI-to-USB cable is working, and Windows can see the device. I don't have a MIDI-to-USB cable or Windows 7, so I'm using an Axiom 61 keyboard controller without a special driver, and Windows 10.
Start MuseScore and go to "Edit," "Preferences," "Note Input." Make sure "Enable MIDI input" is checked. Go to "I/O" and select your desired "MIDI Input" device with the dropdown field. Keep in mind that it might not be identified as a Yamaha PSR-270. If you have only one MIDI device connected to your computer then it should be selected by default (at least, my Axiom 61 was). Then click "OK."
You might want to close and then restart MuseScore at this point, since the "I/O" tab says that any changes made on that tab require that you restart MuseScore-- although I didn't actually have to restart it.
In the blank score, click on the measure where you want to input notes, which will draw a box around that measure. Then click on the "N" icon that's at the beginning of the note toolbar (underneath the "Create new score" icon). Then you should be able to input notes by pressing keys on your PSR-270. The lengths of the notes will be based on the type of note which is selected in the toolbar, and you may accidentally get multiple notes on the same beat if you play the keys too fast-- I'm not sure, but I think that might happen when you play a new note before releasing the old note (i.e., legato style). Of course, if you want to enter multiple notes at once-- e.g., a chord-- then you can do that.
I hope this helps you to get it working. If not, let me know and we'll go from there.
