There is one Sigma 24-105 lens listed on the Adobe-Lens-Profiles page, that doesn't say Art it says A013. If that's the same lens, then it was added to LR 5.4 about a year ago:
What version of LR are you using? Use Help / About LR or Help / System Info to find out. And what OS are you using?
I am using the latest LR6 on Windows 8 64 bit. If it is in LR 6 where can I find it? I've tried looking in the Lens profile in the Development module, but it has very few items for Sigma. Being new to LR, maybe I'm looking right past something obvious?
On another note, I've downloaded the Lens Profile Downloader, as deepakgahlot suggested, but I am confused about how to locate the Sigma lens I want. It wants my camera model, but doesn't have the D750 on the list. I thought I would be able to scroll through the lens down to Sigma lens, much like Ssprengel pointed me towards, select and download. This is not what I am finding.
Any help on either of these queries would be appreciated.
Lens profiles that Adobe supplies are almost always just for raw format files. If you don't see the lens profile then you're either viewing a JPG from the camera (or TIF or PSD from PS) or your Art lens is different than the A013 lens that has a profile.
For a raw file this is what I see for the Adobe-supplied profile for a Sigma 24-105 f/4 lens, four camera manufacturers and for a D750 you'd pick the Nikon one, unless you have a Canon 750D then pick the Canon one:
For the lens profile downloader, just select the camera manufacturer and look for the lens that matches. The camera model doesn't have to be the same and most likely won't be. The only restriction is that you should not use a crop-frame camera lens profile with a full-frame camera because the crop-frame data doesn't extend far enough across the full-frame image to correct things near the outside and the corners.