Hello, and welcome to Mentor's updated community space!
The new interface is quite different from the old one. One of the useful new features is the ability to "follow" the communities whose content you're interested in so that you can view only the information you want to see in your Activity Stream.
To follow a community, simply click the Follow button on the top right of each community page:
The updates from each community you follow will then appear in your Activity Stream, which can be accessed by clicking your account name at the top right and selecting Inbox & Activity. The default stream is called Connections Stream, but you can add new streams and organize your information as you'd like.
Following a community will allow you to see posts from its subcommunities in your Activity stream. You will not get notifications of announcements from those subcommunities, however, so you may still want to subscribe to those separately.
And that's it! Hopefully this short guide has been helpful. If you have any further questions about the new community interface, please don't hesitate to ask.