Choose Swatches from the Window menu.
From the Swatches palette's flyout menu, choose one of the Pantone options, such as PANTONE solid coated as shown here.
Photoshop will ask if you want to replace the current batch of colors or append (add) the new group to the existing batch.
Here, they are shown appended. The colors at the top are Photoshop's default colors and below that is where the Pantone group starts.
See the stair-step pattern in the palette? If you make the palette larger the colors will line up in groups, making them easier to scan.
Or, you can go to the flyout menu again and set the palette to display as a list.
And here it is in list view. Below the last of Photoshop's default colors (the various browns) you see the Pantone colors start. The numbers are shown which makes them easy to find in a Pantone swatch book.