If you're having trouble with setup, there are some basic ways to investigate.

I can't tell if StreamOnce is enabled in my community

If StreamOnce is enabled, you should see Google Groups, Microsoft Exchange, and other stream integrations in the list when you edit the place template for a group you own and click Add Stream. If you don't see any of these integrations, the StreamOnce Add-on may not be enabled. Community managers should be able to see StreamOnce in the list of Available or Installed Add-Ons on the You avatar > Add-Ons page.

I can't edit place Activity page

You need to be an administrator of a group to add streams to an Activity page, which means you need those same rights to set up StreamOnce. Check the People tab of the group to see the Owned By list on the left, which shows the list of group administrators. If you need to get administrative access to the group, anyone in this list can add you by using Manage > Members.