The raw format of Nikon D7100 is not yet supported by Aperture, sorry. You will have to wait for the next raw support update.
See this for the list of currently supported cameras: Apple - Aperture - Technical Specifications - RAW Support
Send feedback to apple, that the raw support is needed - you are not the only one waiting, see the "more like this" to the right.
Apple - Aperture - Feedback
Regards
Léonie
The usual work-arounds:
- Shoot raw+jpeg while you are waiting and use the jpeg as original, then switch to the raw original later
- or use the camera software to develope the raw
- or use Adobe's free DNG converter.