sorry yeah my first (old wallet) was for mac new one is win
to find your wallet.dat on mac:
1. Right click Finder
2. Press Go to folder
3. Type: ~/Library/Application Support
4. Locate your bitcoin folder
5. Locate the wallet.dat ... either delete it or store it. I highly recommend storing it or importing the private key later just in case
6. Put your wallet.dat in there, and there you go
I also had this issue, every time I added a new wallet.dat I had to completely re-sync my bitcoin client which was a pain
I'm unsure if this occurs on windows, but it does occur frequently on mac. I switched to multibit as its much more lightweight and easy to import!
good luck