When you are in the platform, navigate to and click on the Challenges link on the left panel on the platform.
You will see a couple of tabs at the top: All and Joined (note that if you are an Admin, you will also see a third tab: Template).
Clicking on All will show you all Challenges available in your company. Clicking on Joined will show you all the Challenges you have already joined. You can further filter each page by choosing either by showing All, Active, Complete, or Upcoming using the drop down just below the tab headings.