Searched a while and found a lot of people who had the same problem when trying to activate the Gravity Form plug-in.
"this plugin doesn't have a valid header"
The problem in my case was that Safari (on Mac) extracted the zip-file and forced me to archive the folder "gravityforms" my self. This does not work.
You probably have to upload the original file.
So disable the "Open 'safe' files on download" (not sure what it's called in english) in Safari's settings and download again. Now you should be able to upload the plug-in.