Click on your icon at the top right corner of the screen.
Look at your profile. It states that you are accepting all challenges.
If that's not the case, you need to click 'My Settings' and customize your account to match your preferences.
If you do make changes to your settings, make sure you go to the bottom of the page and click the 'update' button, or nothing will be changed.
Take some time to look at all of the setting options. You may find other things that you wish to change.
EDIT: <You could also just go to the bottom of the page and click "Site Map". There are many links on that page that should help you.>