If your shapefile or data frame is in lat/long GCS, you could do this pretty simply and quickly:
1. Add two float fields - call them lat and lng, or something else if you prefer
2. Right click the field heading, select Calculate Geometry. Select either X point or Y point from the dropdown as appropriate for that field. X = longitude, Y = latitude.
3. Select whether to use the shapefile coordinate system or the data frame one.
4. Click Ok, and you're done
This won't work to give you lat/long if neither the shapefile or the data frame is in a GCS. If both are projected coordinate systems, the results will be in that as well. You might try adding your shapefile to a new, empty map where you've first set the data frame to whatever GCS you want to use, then do the above. Your existing map will still be fine, but your shapefile will then have lat/long attributes.
Technically, maybe you could just switch your existing data frame to GCS, perform the above, then switch back, but I like to keep the process cleaner.