The command doesn't have to be shown on the screen when you invoke it. On the main menu screen you can use the arrow keys to change which line of the central menu is highlighted. As you use the arrow keys to change the highlighting, you also change which command has the square brackets in the lists at the bottom of the screen. When you select a command from the main menu, pine displays a different screen. On each screen that pine displays, the bottom two lines show command names and with each one the single keystroke that invokes it.
Different screens offer different selections of commands.
The message you will view is the one which is highlighted. Notice that the message number of the highlighted message is shown in the upper right part of the screen. You can select a message by using the arrow keys to move the highlighting to it. I should probably warn you that the message number that pine is using is just the position of the message in the index. The same message can have a different number next time you run pine.