How to show the map in Photos?
For more recent versions of Photos.app (Version 2.0 that came with macOS 10.12 Sierra at least), there is a "Places" item avaialble under "Albums" in the sidebar (use "View" -> "Show Sidebar" if it is not visible). This gives a map view similar to that available in iPhoto.app.

For older versions of Photos.app, it does not appear that there is a way to see ALL photo locations on a map at one time, aside from a small Info window. Here's what you need to do:

For the small info window that shows all locations:
 1. Go to the Albums tab in Photos
 2. Right-click on "All Photos" -> Get info
 3. Note the small window has a map included which you can zoom in/out of

For maps that vary by date:


Go to View menu -> Metadata -> Location
Go to the Photos tab and zoom all the way out to Years
Note that there are locations next to the year. Click on the location name