That field just under Search.

I guess I must have set something wrong in the ACP, but cannot find back to it.

That is your default activity stream. Go to Activity on the main menu, select Unread Content to set it then click on the check mark next to it.

Thanks.

I want to change the default choice for all members, but I cannot find the setting in ACP.

in case anyone else is having the same problem, I asked Support and they answered:

Quote

To get back the "Unread Content"link, go to ACP -> Streams -> edit "Unread Content" and scroll to the bottom of the page, then enable "Default Stream", and save.