I think when you initially search, it'll only be searching "Communities" (look towards the upper left, under the word "Search", at the drop-down tab menu). Change that to "all" and it should repopulate below. Click on the link which has the "...NameOfCommunity... - ## subscribers" formatting, and it should take you to the screen where you can subscribe to the community to the upper right.

I would think searching with the "Communities" in the tab would work, but I'm guessing that means communities which are only in the community or instance that you are currently in. I hope that makes sense.