Do you want the captions permanently written onto the image, or do you want them to be overlayed on the photo while browsing the imaes in Aperture?
- To add the captions as part of the image file you will need an external editor or a plug-in - Aperture does not do compositing. Use the versatile BorderFX plug-in (download from the BorderFX's site. ), the free Gimp, Photoshop, Pixelmator, or simply Preview.
- To see captions while browsing, enable Metadata overlays and edit the Metadata presets, or enable captions in a slideshow (in the settings for individual slide).
See this manual page for how to display the metadata with your images: Displaying Metadata with Your Images