If you are unable to edit tag information in iTunes it is likely that the files are marked as read-only or your account does not have the correct security permissions to make changes. This can happen when a library is moved between computers or accessed by a different account on the same computer. The error message "The iTunes Library.itl file is locked, on a locked disk or you do not have write permission for this file" can also be a result of permissions problems. The following process should install the correct permissions.
Right-click on your main iTunes folder (normally in User's Music unless you have relocated it) and click Properties, then go to the Security tab and click Advanced. Use the Change Permissions... button grant to your account (or the Users group) and SYSTEM full control of this folder, subfolders and files, then tick the option to Replace permissions entries... which will repair permissions throughout the library. Note also that the Administrators group should normally have full control, though iTunes should work properly either way. When the permissions have been applied switch to the General tab of the folder properties dialog, click in the Read-only check box to clear it, then click Apply.
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If you don't have the option to change the permissions then use the Owner tab and Edit... button to take ownership from an account with administrator privileges. Tick the option to Replace owner on subcontainers and objects.
Repeat with the media folder if it isn't stored inside the main iTunes folder and/or with any other folders that you may have imported media from while leaving it in the original location.
If you have brought over the library from another computer, and you don't plan to use the library on both computers in future, then you may also want to remove any "mystery" identities (S<string of dashes & digits>) that have come over from the old computer.
Related support documents:
TS1277: iTunes: Missing folder or incorrect permissions may prevent authorization (Document retired, archive version)