At the beginning of my fresh installation it worked great, but now it doesn't change at all.
How do I "re-enable" it or "fix" it?
If u want to cancel the activity in the dash just go to System configuration and then to privacy settings and cancel the recent activity .
I suppose that by "it doesn't change at all", you mean that the Dash Home is not updated, even when you type a search.
If this is the case, there are two things you can do. First, wait a little more. The Dash may need some time in the beginning to index everything and then to refresh. Second, you can try logging out and in again.
If those two actions don't resolve your problem, you may have another problem.