I have a K750i so it should be pretty similar.
Mine has a desktop icon, 'Sony Ericsson File manager'. Presumably you have something similar. If so:
Open the manager, you should have an explorer type window open. Navigate in the left pane to the folder or file containing the files you want to copy to your phone. These should display in the right pane. Find the file you want to copy across, highlight it, right click, then click 'copy'. Now navigate to your phone memory in the left pane, and find where you want to put the mp3s in your phone. Once you have it highlighted, click anywhere in the right pane (which will also show the content of the folder in the left pane) and left click in a blank are and select 'paste'.
That should copy across the mp3 from your pc to the phone.