Adding a field to a table doesn't automatically put it on the layout. The layout is your artwork...
Now that you've created the field, go into layout mode and add it to the layout, formatting it how you desire.
FileMaker can add the field automatically to the current layout if you have selected that setting in preferences. I leave that option disabled as FileMaker just sticks the fields at the bottom of the body, resizing the body if necessary and that generally messes up my layout so it's less work for to leave that option disabled and to then use the field tool to add the field to my layout where and when I choose to do so.