How can mouse support be enabled in terminal Emacs?

Hit F10 to open the menu and use the arrow keys to navigate to “Options” → “Customize Emacs” → “All Settings Matching…”. Type mouse and Enter.

If your Emacs version doesn't have a menu when running in a terminal then run M-x customize. (This means: press Alt+X, type customize and press Enter.) Navigate to the search box, type mouse and press Enter.

Mouse support is called “Xterm Mouse mode”. You can find that in the manual. The manual also gives a way to turn it on (for the current session) — M-x xterm-mouse-mode.

In the Customize interface, on the setting you want to change, press Enter on “Show Value”. A “Toggle” button appears, press Enter on it. Then press Enter on the “State” box and choose either 0 for “Set for Current Session” or “1” for “Save for Future Sessions”. (You can choose 0 for now and come back there and choose 1 later if you're happy with the setting.)

add (xterm-mouse-mode 1) to your init.el file