I just bought this game and I also downloaded the free "Arcane Unleashed" DLC, but I can't figure out how to install it since it's a .PKG file rather than an application. Do I just need to put it in the game directory (and if so, which folder)? Also, do I have to start a fresh game with the DLC already installed, or will it work with pre-existing saved games?

PKG-Files are for Mac. You must change the System to Windows.

I have a problem then, because that's not showing up in my game library. At the bottom of the list of offline installers (I don't use Galaxy) there's a section for "DLC installers" that lists a single download labeled "Arcane Unleashed". It's listed as 1 MB, but the actual download is only 16 kb and is named dlc_pathfinder__kingmaker___arcane_unleashed_enUS_2_0_7b_32481.pkg. Sounds like an issue I need to report to GOG, unless you have any suggestions of something else I could try?

Also, can anyone confirm whether or not it's necessary to install the DLC before I start a new game? I just checked, and I have the system set to Windows, same as it's always been.

Just curious: does anything for the DLC show up under "GOG Downloader Links"?

Hmm, good question. It does in fact list the Arcane Unleashed installer (three different versions in fact, for Windows, Mac and Linux). I'm currently at work, however, so I'll have to wait until I get home to try it. (I had GOG Downloader installed on my previous laptop, but not on my current laptop, which is why I haven't tried that already.)

Update: I got a very quick response from GOG Customer Support. They asked the same question about whether I'd selected the correct operation system. I tried changing it from Windows to Mac and back again, and after that I was able to download the file properly. Still don't know what caused the problem in the first place, but it seems to be resolved now.

I can confirm the same issue; I have never downloaded GOG games for Mac, yet by default a DLC I purchased was always downloaded as PKG file. Switching the download links from Windows to Mac and back to Windows solved this, so there must be some issue on GOG's end.

This is definitely a bug on the GOG site. Happened to me with several games, also on past computers/ Windows installations. Just always select the OS of choice, change to another OS and then back to the OS of choice. Not a big deal, but quite annoying sometimes...