There is to options to calculate XY or Latitude Longitude Coordinates values using ArcGIS Calculate Geometry Option or ArcToolbox.
1st Option :
1 Add a new fields to the attribute table, one for Latitude and one for Longitude.
2 Right click the field header select Calculate Geometry and pick either the Data Frame CRS or Data CRS to append the coorindates. (bear in mind the values will be in latitude and longitude as this is a GCS).
2nd Option :
Use a tool which is called the "Add XY Coordinates" tool you can find this in the Data Management toolbox. The tool adds two fields called "Point X" and "Point Y" and populates them with the coordinate values (bear in mind the values will be in latitude and longitude as this is a GCS).