raikoification if you're new to Construct, make sure to only use the latest stable releases. Beta releases can contain bugs, and are not as thoroughly tested.
To run offline, you need to open C3 and wait for a notification to appear in the corner saying C3 is ready to use offline. If you close it before then, it hasn't finished downloading everything it needs so won't work offline. Once you see the notification, visiting editor.construct.net will work even if you are offline.