How to enable iTunes Menu Bar on MacBook Pro

iTunes 12: Display menu bar - https://discussions.apple.com/thread/6784463

iTunes Store: Authorize or deauthorize your Mac or PC - http://support.apple.com/kb/HT1420 - and another helpful post: https://discussions.apple.com/thread/3798085?answerId=17828050022#17828050022

Note distinction between computer authorization and device association - https://discussions.apple.com/thread/7106883

A better way to get your library onto your Mac:

Moving a library: Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its sub-folders and files) intact to the other drive, hold down the option/alt key while starting iTunes, and when requested to select a library select the iTunes Library.itl file in the copied folder.

When this is to a new computer put the copied iTunes folder in the default location of Macintosh HD > Users > *User Name* > Music (or Home > Music) then you don't even need to start with the option key held down, iTunes will automatically look for it there. (Make sure there isn't anything already in the iTunes folder there that you want to keep since you will be replacing it with the one you are moving.) Remember to authorize the computer in iTunes to your Apple ID if you use this to sync to Apple mobile devices or play DRM protected media (e.g., movies).

Certain items used in iTunes related activities are not stored in the iTunes library folder but in special folders. If you are moving a library to a different computer you may wish to copy these too:

- Apple Music cache files: http://dougscripts.com/itunes/2015/09/new-flush-apple-music-cache-files/

- iOS device backups, various preferences files: https://discussions.apple.com/thread/3285925

Windows users see tip at: https://discussions.apple.com/thread/4095896?answerId=18879381022#18879381022

Consider moving everything from your Windows PC to your Mac in a single move, including your iTunes library: About Windows Migration Assistant - http://support.apple.com/kb/ht4796 - Learn about how to use Windows Migration Assistant to migrate your files from a Windows PC to a Mac. Additional tip by ChrisCA 05/2014 - https://discussions.apple.com/thread/7022914?answerId=28178940022#28178940022

If iTunes window is in full screen mode, moving cursor to the top of screen brings down menu bar. The menu bar can also be hidden in "regular" (not full screen) mode, for all apps. To control if menu bar is hidden or shown, go to System Preferences General pane; there is a checkbox for Automatically hide and show menu bar. If that setting is NOT checked, the menu bar is always visible (for all apps) when the screen is not on full screen mode.

"Authorization" is for the computer (not iTunes library) to play content purchased using a particular Apple ID. One computer can be authorized to play content from multiple Apple IDs. One Apple ID can be authorized with up to 5 computers.

how do I authorise my Mac to accept my iTunes library which I am running on 2 x iPads, 1 x iPhone and a PC.

I'm not sure what you mean by this question. If you authorize the Mac for the Apple ID used on your other devices, and you sign in with that Apple ID in iTunes, you can download and play content purchased (with that Apple ID) on the other devices. But this is not directly related to the "iTunes library" on the other devices. Apple ID and authorization is for purchased content from iTunes Store.