I just downloaded and synced clam wallet then imported bitcoin wallet.dat file. Then did the same for litecoin and dogecoin wallet.dat files.
Balance is still zero clams.
Is there any other step I have to do or simply those addresses were empty on May 2014 (can't remember for sure)?
You can actually see if the addresses were empty: look in the block explorer for the dates. This makes it much faster/easier to follow.