Try restarting the computer. If that doesn’t help, reset the preferences.

… when I click on the custom line tool, the icon is highlighted, but absolutely nothing happens or pops up, although I'm told there should be a menu of some kind. Relatively new user, so I've never had to use the custom line tool before …

To make the Smart Line Selection appear, try holding down the CTRL key while clicking on the Custom Line Tool icon (= hexagon icon).

Holding CTRL worked! Thanks a bunch!