You have to click on the little icon in the bottom right corner of the Curves panel to make editing possible.
If this is color film, I'm not sure how good a conversion this would be, it will certainly require some tweaking of the colors, because of the orange mask in the negative.
And DNG files from a scanner? I doubt that they will be true raw files ...
You can scan a tiff with all shadow and highlight detail present, and that will be as good as any raw file.
Thank you so much Per. That was very simple! Somehow that important point was missing from the tutorials I saw. Also thanks for the comments about TIFF vs DNG files.
If this is color film, and your scanning software does a reasonably good conversion, I would scan them as 16-bit tiffs.
If not, scan them as positives, and do the conversion in Photoshop, which has a preset called Color Negative in the Curves dialog.
It does a remarkably good conversion with scans from my Imacon Precision II scanner - the color is spot on, but I have to adjust the end points of all channels to retain shadow and highlight detail. The results with other scanners (Epson) are less good, but it's still possible to get decent results with some tweaking.