If you can't see your organisation's challenge this will probably be because you haven't set your Organisation in your profile.
You can navigate your way to your profile details by tapping on the 'Settings' cog in the top right hand corner of the app. From here you should see an account overview with an Edit profile field, tapping this will take you through to your profile screen with various fields.
Navigating your way to the Organisation field you should be able to then add in your workplace - this is an auto-complete field so this should allow you to find the correct organisation. If you navigate back to the Challenges dashboard you should now see your workplace's current challenge as well as any archived challenges.