not 100% sure, but I think you need it in this format, first the private key then the utc date, in one line.
Example (don't import this one, its the brainwallet privatekey from: correct horse battery staple and lol):
5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS 2011-12-31T16:42:00Z
5HsQCyqCw61VPQ9tN4ttasHW3AH6cJ6tJdneAXR8gUJ2MWyxxpA 2011-12-31T16:42:00Z
I guess the date is used to scan the blockchain from this point, so just set some time before
the address received the first coins.
Note: If your private keys are "compressed" they are shorter and don't start with 5, but it should still work the same.