Hi
Please try to do add it this was:
- right click on LXPanel
- click on "Add/Remove Panel Items"
- click "Add"
- search for "Menu" and select it
- click "Add"
If the plugin "Menu" is not in the list, it's not correctly compiled. But there are several threads in the forum on how to do this

, but first try it this way. If it doesn't work, be more verbose please. (Distribution, Version of LXpanel ect)
maces