In your case, it would be better to have transferred the library to an external drive from the PC. Then, connect it to the iMac and put the library onto the Music folder.
You used Migration Assistant, and it creates a new user with the transferred data.
Open > Log Out, and log in the new user. Then, open a Finder window, choose Music in the sidebar and you will see the iTunes library. Now, you have to move the iTunes library to /Users/Shared (select Go menu -in the menu bar- > Go to Folder, type that and copy the iTunes library to it). Finally, log in the other user, go to the same folder and move the library to the Music folder.
After doing that, open System Preferences > Users and Groups, choose the user created by Migration Assistant and press the - button to remove it. This is easier if you have an external drive