I hold ctrl key, but nothing happened. What is LMB?
Left mouse button. Hold control while clicking it.
How to use Master Public Key to track all the addresses?
You can find the MPK in "Wallet -> Information". I used
Sentinel on my Android phone, but you can also paste it
here.
When I looked at the blockchain explorer, 18mbtc amount is both in confirmed received and spent. However Electrum add that amount to the balance history. Why?
Aren't you looking at the address of your closed channel? It had to be spent to one of your address so that the channel could be closed. Still, I am not sure why there is a discrepancy between the "History" tab and your total balance in the bottom-left corner.