Freya does not give notification when I plug my USB drive. How do I active it?
You can't, because it was never intended to be possible. Due to security concerns, the elementary devs decided that the default behaviour when plugging in an external drive was to keep it unmounted and leave it to the user what to do with it.
Questionable as it may be, it is a design choice. What I personally find less justifiable is that the volume does not get mounted even if you drag a file on it, which to me is a bug, but it has been currently labelled as "wishlist".