You need to drag the gradient filter over the image in the main editing window. Only then does the range mask pull down menu become active. It’s the same with the brush tool you need to start brushing to activate the range masking feature.
I've just got the latest version of lightroom for mac and I cannot apply range mask for luminescence/colour on gradient filters. Have they got rid of it of it or put it into an obscure place?
In addition to my questions above, 99jon makes a good point ... and so re-reading your statement, it's not clear ... are you having trouble FINDING the range mask, or are you have trouble APPLYING it? Your words imply both, which is contradictory.