If you download the blockchain file from some source, I mean not through bitcoin client, then you must add commandline option "-rescan" when starting bitcoin.

prompt$  bitcoin-qt -rescan

This way the client to read again each block to see if there are transactions with your addresses. If the wallet.dat already "knows" about all it's transactions, then you don't need -rescan, but if not sure, try it. It takes a few minutes.