I've done it before, but i just want to know how you log in on the software?

If by the actual music making program you mean the MuseScore notation program (you previously installed) on your computer, simply open it as you would any other app.

If you have musescore on one computer and you have a score and you log in on another computer will you have access to that score?
Only if the computers share files over a network, or something like the cloud; or you copy the score to a usb stick to open on the second computer.

Regards.

To expand on the above:

MuseScore does not have a concept of "logging in". It is a program that runs on your program, not on the web. if you have MuseScore installed on one computer, then you can do anything you like with it on that computer, including saving files to that computer. If you now go to a second computer and want to use MuseScore there, you need to install MuseScore there just like you did on the first computer. And if you want to access a file that you saved on the first computer, you need to find some way to copy that file from one computer to the other - like using a USB driver as mentioned. Or, an easier way if you do this a lot, use a cloud service like Google Drive or Dropbox and save the file there instead of only on the hard drive of the first comptuer. Then the file will be available to all computers that sue that cloud service.