Can I transfer my saved game between platforms?

Your VAB and ship files should be in ~/Library/Application Support/KSP_osx/saves/[YourSave]/ships/ on OS X.

The location of the real Library folder is a bit different on Mountain Lion (OS X 10.8), you will need to use Finder to get to it.

• Select "Go" (Or press Command + Shift + G) from the system menu

• Then select on "Go to Folder" (skip this step is you used the keyboard shortcut)

• Type in "~/Library/Application Support" and then "Go."

On Windows, the same files should be in the folder at %KSP_ROOT_Directory%\saves\...

Yes, this is actually fairly easy. All of your save data is stored in plain text files as a subdirectory of the saves folder in the main KSP folder. Each save game has its own folder, which includes subdirectories for VAB and hanger ships. Simply copy the desired folders (be it the entire save or just a few .craft files) to the new install location.

I'd like to note if you have KSP on Steam on OS X, here's where the ship saves live:

~/Library/Application Support/Steam/SteamApps/common/Kerbal Space Program/saves/<save name>/Ships/

Note the spaces in the path.