If you're lucky, it'll be supported in the next release, due any day now, although it's not included in the RC version which has been around for a month.
If you're not, you'll have to wait another 3 months - unless an RC becomes available in the meantime.
I suppose the only workaround is to use the Nikon-supplied software. I'm sorry if that was obvious.
I've always wondered why the camera makers don't work more closely with Adobe so that supporting software can be ready before the camera is released. You'd think this would leverage greater sales and breed more customer satisfaction. On the other hand, none have embraced the DNG format, so maybe they just dislike Adobe.
You'd think so. But maybe they're more interested in pushing their own conversion software?
I'm in the same situation.
- I could use NX, but then I would end up with jpegs or tiffs, neither of which is my permanent storage format (DNG), nor a viable alternative (not being raw).
- I tried to install Adobe DNG converter, but it prompted me to install their app manager, which then said it couldn't install DNG converter. (DNG converter 220.127.116.11 does not recognize the D5100's NEF files.)
- Preview and Quick Look don't recognize them yet; no OS X support.
- Xee does open the D5100's NEF files. (Version 2.2 is current from http://wakaba.c3.cx/s/apps/xee.html .) It is designed for viewing, not editing; I use it for my first preview/cull step (sometimes).
- The GIMP also opens the D5100's NEF files. (Version 2.6.1 from http://www.gimp.org/ , which says it uses UFRaw 0.17 which in turn uses DCRaw.) However, no option to save as DNG.
A couple options you could try that will probably also be unsatisfactory:
1. Alter the 5100 raws to look like they're from a different camera. Only works if the 5100 sensor has similar response to the one you're emulating.
ExifTool can do it, and RC Importer can automate it upon import.
2. In NX2 - just do the bare minimum of raw conversion - such that the same settings applied to the tif can be reapplied to the raw, once supported, to get the same results. - I used DxO and Lr2 this way for a while - normalized DxO output (DNG format - so it still looked like a raw to Lightroom, despite being cooked) such that the same settings applied to the DxO output resulted in almost identical look as those settings applied to the raw - that way I could pick either. I used DxO as front end when I needed lens corrections or better noise reduction, otherwise Lightroom. You could do the same thing with NX2, although the NX2 output will always look cooked to Lightroom, I think, so that would have to be taken into account...