When viewing a message you can select "More" -> "Tag" and then click one one of your tags to apply that tag to that message. But clicking it also hides the menu, forcing you to start over with "More" to add/remove additional tags. One of the main benefits of tags over folders is that you can use more than one on the same message, so it feels like the design should expect you to interact with more than one tag by not closing the menu until you click away or hit escape or something.