In the print composer you the map has to be selected. Now Go to the Grid section and this is what you normally see:
Deefine a new grid (plus icon) and in the CRS section choose change. Select the one you want, for instance EPSG 4326. This is what you will have:
The only thing to do is set the appropriate x and Y interval. Notice that the intervals are set in Map units. In order to show lat lon coordinates you will have to declare the interval in decimal degrees. For instance, lets say you want a lines every 5 minutes. The value for the interval will be 5/60=0.0833333333333333. The value for the interval. Also if you want to show degree, minute, second with suffix do not include decimals for the seconds (Draw coordinates/coordinate precision).