Your app is not updated to the latest version. Try these two things.
Also try this.
In chrome browser, goto, chrome://appcache-internals/
There you will see app.mindmapmaker.org Remove that item.
Then goto app.mindmapmaker.org again. It will then install the updated copy of the app.
Let me know if it works. (File Save & Drive saving)
Also try this. Inorder to delete the old version of mindmaps from your browser.
Click, Chrome Menu->More Tools->Clear Browsing Data...
Click on 'Advanced' and select 'All Time' from Time Range.
Then select all items in the box to properly remove the mindmap app from your browser.
It will take some time to clear the browser. Then relaunch http://app.mindmapmaker.org
Check if the app is okay now. (File Save & Drive saving)
To save in drive, click Save As->Google Drive. Then click 'Authorize'. After clicking 'Authorize' it should bring a new window where you can authorize Google Drive. Let me know if it works.