If a Leader is attached to a unit, the menu options are not shown.
Therefore you should use (and learn and remember before) the menu options shortcuts.
The important shortcuts are also in the "Chart" window of the Vassal module.
To attach/spread a Leader from a unit, just doubleclick on the unit.
But my suggestion is, go online and play only one game with a familiar and experienced Vassal player.
So you would see and learn how the Vassal module is used (and the few differents to the live boardgame)
at least 10 time faster as you will do in 50 solo games