Tools -> Import Private Keys

Create a file with the private key inside it. If you do not add the time of creation of this address, you get this message:

"There were missing dates. Have to go back to genesis block."

I think this date should be the first transaction date registered in the block chain.

Key createdAt is in UTC format as specified by ISO 8601 e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory

To be safe, turn the date back a few days.

Importing private keys into MultiBit:

1. Create a file with the extension ".key" (like "mybitcoinkeys.key")
2. Put each private key on a separate line.
3. Next to each private key, type a space and then the date that the private key was created (in UTC format).

(this last step is important because MultiBit will only check the blockchain from that date forward)

Here's a sample file that I dug up:

# KEEP YOUR PRIVATE KEYS SAFE !
# Anyone who can read this file can spend your bitcoin.
#
# Format:
#   <Base58 encoded private key>[<whitespace>[<key createdAt>]]
#
#   The Base58 encoded private keys are the same format as
#   produced by the Satoshi client/ sipa dumpprivkey utility.
#
#   Key createdAt is in UTC format as specified by ISO 8601
#   e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory
#
THIS_IS_THE_PRIVATE_KEY 2014-08-01T00:00:00Z

# End of private keys

P.S. The date can be as early as you want but should absolutely be before the first transaction involving the address. The time of day doesn't matter because MultiBit will check starting with the beginning of the day anyway.

Importing a private key into MultiBit Classic is not hard at all.

Open a text file and copy in the following line:

5JinrCK2zeijdidLmjxCfpRWMbfXNS8i6XdUbmQAvW7CC55LQcU 2014-08-01T00:00:00Z

Replace my private key with yours, leave a single space after the key, and don’t change the date, leaving it as 2014-08-01T00:00:00Z

When saving the file replace the extension .txt with .key.

Open MultiBit Classic and import the key, and you are done.