Q: how to transfer music from Mac to my iPad

Here's how to do it using the Manually manage method.  Connect iPad to Mac.  You should see the iPad's small device button on the horizontal bar that goes across iTunes window. 

Check something first...  Click iPad's device button, to show its settings screen in iTunes.  On iPad's Summary settings screen in iTunes (what you see now), there is a setting (a checkbox) for Manually manage music and videos.  Checkmark that box, if it's not already.  If you don't see that setting (you see one that says Manually manage videos instead), please post back.  If you made a change, click Apply button.  This confirms your iPad is set up to load songs manually, instead of using one of the other methods.

To manually load songs on iPad, show your music library in iTunes (click the Music button at left end of horizontal bar).  Near middle of horizontal bar, click Playlists.  A sidebar appears along left side of window, with the iPad listed under Devices.  You can change your "view" of the music library, using the control at right end of horizontal bar (I like using the Songs view but you can view by Artists or Albums).  From the music library, click-hold and drag songs to the sidebar, and drop on iPad.  You can do this with one or more items selected.  Or, instead of dragging and dropping, right-click song selection and (from the menu) Add to Playlist -> [name of iPad].

To see what is on the iPad, in the sidebar, click small triangle to the left of iPad name, to drop down the iPad's content list (indented below iPad).  Click Music there, in the content list.  To the right, you see the list of songs on iPad.  NOTE:  To remove a song from iPad, select it on this list and press Delete on keyboard.  You can do this with one or more songs selected.

To load other types of content manually, do the same thing except show your Movies (or TV Shows, Podcasts, etc.) library instead of Music, and drag those items to the iPad in sidebar.

Being 90 doesn't automatically exclude a person from being smart.  I know some of the top helpers on this web site have mentioned being retired.

https://discussions.apple.com/thread/7163823 - If you have signed up for Apple Music or iTunes Match you won't have the option to manually manage or sync music to a device in iTunes. Music syncing is done through iCloud and all your eligible music should appear there. Movies can still be synced.

A bit more about this, and some options- https://discussions.apple.com/thread/7194995 and: Move content from iTunes on your computer to your iPhone, iPad, or iPod touch

- https://support.apple.com/en-us/HT201593

and a workaround - https://discussions.apple.com/message/28842873#28842873

If you have not signed up for one of the iCloud based services:

https://discussions.apple.com/message/28481661#28481661 - "Music syncing has changed in iTunes 12.2.0 and iOS 8.4 Music App."