Open Canvas and click on "Commons" in the left panel.

If prompted, click the "Authorize" button. You only have to authorize the first time you open Commons. 

You now have access to the shared resources in the Commons.

Canvas

Some Canvas features require the user to authorize access. The steps below show how to authorize these features. This example show how to authorize the Canvas Commons but can be used to authorize other features that present the Authorization window.

Canvas - Authorizing Features