Thank you, but how come I am not able to see the private key for those pre-built 5 accounts? does it mean, I don't need private keys for those accounts?
The private-key is inside your Ledger. Putting in a simple way, MEW uses your public key (address) to see your balance and build the transaction, which its sent to the Ledger device, signed with the private-key and sent back to the computer, where its broadcasted. The private-key never leaves the device (reason why you can't see them, and why hardware wallets are so secure compared to normal desktop wallets).