I have upgraded my multiBit classic to multiBit HD but I am having hard time to find how to import private keys to my multiBit HD wallets. Any idea ?
All your addresses and private keys in MultiBit HD are produced from the wallet words that you are given when you create a wallet. (This is why it is so important to write your wallet words down).
Because everything is generated from one thing the randomly generated private keys in MultiBit Classic cannot be imported into MultiBit HD.
The safest way to transfer your bitcoin is to spend them.
There is a step-by-step guide on migrating from MultiBit Classic to HD in the help here: https://multibit.org/en/help/hd0.1/how-to-upgrade-from-classic.html