RAW data from the following Sigma cameras is currently supported:
If your camera is among these, then simply import the photo as usual. https://helpx.adobe.com/lightroom/help/importing-photos-lightroom-basic-workflow.html
sigma dp1 merril.
When importing writes the files appear to be unsupported or damaged.
But sigma photo pro opens without problems.
I'm not absolutely sure - maybe somebody who knows this for sure can jump in here -, but isn't the Merill an updated version of the FOVEON sensor?
If yes, this seems not yet supported in LR and ACR.
Of course, Sigma own software can open theses files. As a workaround, you may export to TIFF in the Sigma software and import the TIFF into LR until your camera is supported.
I too cannot see Sigma .x3f files in my LR6. Camera dp1, Mac Pro with Mavericks. All say this can be done, but it doesn't happen. I prefer to keep everything in .dng and Adobe should push to make this possible for their standard format.