It might be that the file you added was not added to the project build. Click your project > App target > Build Phases
And check that the file exists in Compile sources (if it needs to be compiled), otherwise check Copy bundle resources. If the file does not exist there, drag it there and it should be fine, (from what I understand of your question).
EDIT
You can also check wether a file is added to the target by clicking the file and opening the “File Inspector” (View > Utilities > Show File Inspector, or ⌥ + ⌘ + 1), and check the Target Membership section.
Example:
