Click on UserCP on the blue band near the top of the page. Then look for the heading "Networking" on the lists on the left side of the page. Under that heading you should see "Group Memberships". Click on that and it will bring up a list of groups available to join. If your membership is active, you'll have a "Gold Members" group. Click on the button between that and "Identify me as a member of this group" and save.
International memberships and PayPal paid memberships have to be added manually, and may take a few days to be activated.