You must first accept the challenge invite in Polar Flow (Settings > Your profile tab).
After you've accepted the invite, your data is transferred to the challenge when you synchronize your Polar device.
Your training data will be visible in the challenge within a few minutes after every sync.
The daily activity data is transferred to the service after 24 hours. Therefore you can only see activity data accumulated prior to the day you synchronize. You will not accumulate any activity data during the day you accept the challenge invite.