Due to the recent shift from a Subversion to a Git repository, you'll need to use the latest versions of the Mission Planner to ensure that you'll be able to get the latest ArduPlane and ArduCopter firmware. Versions from 1.0.67 up will pull from the correct Git repository.
Google Code has currently got a file download glitch, which they're working on. So in the meantime, I've put the latest version of the Mission Planner (1.0.68) in a public DropBox folder here.
UPDATE: The regular Google Code repository is now working again.
Once Google has got the download issue fixed, the Mission Planner will be available in the download section as usual.