Check that you are signed in.  If you are, your name will appear in the left hand side of the green bar at the top of the page.

Check that you are a member of the ladder - only members of the ladder will be able to see challenge icons.

Challenge icons only appear next to players that you can challenge.  Therefore, if you are in first place, you won't see any challenge icons because there is no-one above you who you can challenge.  In this situation, try contacting the people below you on the ladder and asking them to challenge you.

If this hasn't answered your question, please use the Feedback form to get in touch.