Linked EPS files in your Illustrator document will not show up in the Studio Designer window if they do not contain a preview image.
Only when a preview image is saved to the EPS file, the image will show up in Studio Designer. Another way of getting around this limitation would be to save the AI file as a PDF file. This way the PDF will generate a preview image for the linked EPS file. Placing this PDF file on top of a structural design file will show the images in Studio Designer.