The solution to transfer the data from Mac (Lion) to PC (Win7) I have, as shortly describe above. The file and folder structure for the garmin basecamp database is identical for the Mac and PC.
Here the steps I made:
>>Copy the 'Database' folder with everything in it from Mac (Lion)
>>MacintoshHD/Users/'your user name'/Library/Application Support/Garmin/BaseCamp/Database
>>Copy the 'JnxFiles' folder (if you have Birdseye) from Mac (Lion)
>>MacintoshHD/Users/'your user name'/Library/Application Support/Garmin/BaseCamp/JnxFiles
Put both folders on a memory stick or in your dropbox so you have them on your PC.
>>Paste (replace) the 'Database' folder in the PC (Win7)
>>C:/users/'your user name'/AppData/Roaming/Garmin/BaseCamp/Database
>>Paste (replace) the 'JnxFiles' folder in the PC (Win7)
>>C:/users/'your user name'/AppData/Roaming/Garmin/BaseCamp/JnxFiles
Like this you will have on your PC the same data with the same folder and list structure as you have it in Basecamp on your Mac. Even the smartlists are there. The birdseye images will work as well.
If you just export the files you won't have the folder and list structure and you will not have the birdseye images.
I first wanted to copy a backup file, but I couldn't even create one. If I can't sort that problem myself, I will send Garmin an email.
But for now I'm happy with this solution. I just thought I share it. Maybe somebody else needs to do the same.
I actually was hoping somebody has a simple solution to sync between Mac and PC, so you could change things at both ends. But as I said, I can live without it.