I just upgraded to OS X Yosemite with the new Photos application. I have seen screenshots online where we can view photos on a map, however I can't get to that map.
How can I see all of my photos on a map, like in iPhoto/Aperture?
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 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
Actually you can see all your photos on a map, it is just not obvious. Go to Photos and go all the way out to see all your images. You now see location names for all locations you have tagged your photos with, together with year or year interval. Simply click the location name and a map will be shown.