madve2: Ultimately the Install Add-ons menu, which doesn't seem to work for the GOG version, just copies files and creates registry entries based on the LINKSLS.INF file at the root of every CD. For the 5 course volumes, it really is as simple as copying the course files (and in that case, as others have pointed out, the registry entries are automatically created for you if you put them at the root of the game directory), but other discs can have extras like Resorts or Flyby videos, which don't seem to install automatically. (Flybys are named the same for every course, so you can't put them at the root, and Resorts have settings that the game can't figure out from the files alone.) To be fair, these are just vanity, so you may just stop caring and be content with copy-pasting courses, but out of curiosity, I have went through the trouble of installing Congress Country Club manually, and I thought I should share the steps it took, in case some of you find it useful:

0. Using regedit, export HKEY_CURRENT_USER\SOFTWARE\Access\Links LS 1998 to a file (as a backup if things go wrong)
1. Copy CONGRESS\CONGRESS.CRX and CONGRESS\CONGRESS.ENV to the game directory
2. Start the game, then exit. This will create some initial registry entries that we can use as a base for the following steps.
3. Delete CONGRESS\CONGRESS.CRX and CONGRESS\CONGRESS.ENV. (We'll add them again in the next step, but to a separate directory.)
4. Copy the CONGRESS and contour folders to the game directory
5. Move all the .rec files out of CONGRESS into to the game directory root
6. Export HKEY_CURRENT_USER\SOFTWARE\Access\Links LS 1998\Options\Resorts and HKEY_CURRENT_USER\SOFTWARE\Access\Links LS 1998\CRX\CourseX where X is the largest number in the CRX subtree. (So e.g. Course5 if Congress is the 5th course you have.)
7. Create a new .reg file using notepad, with the contents below. The first entry is new, so you can copy it as-is, whereas for the 2nd and 3rd entry, compare them to the files you exported in the previous step to make sure you don't accidentally remove any data that you don't want to. (E.g. if you have other extra resorts installed, you'll want to keep them in the Resorts list.) If you have a vanilla installation, this example should work as-is.

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\SOFTWARE\Access\Links LS 1998\Options\&Congressional C.C.]
"Tour"="contour\\tour.ls"
"TourCDVolume"="CONGRESS"
"TourCD"="Congressional Country Club"
"Icon"="congress.flc"
"IconLZ"="contour\\congress.ls"
"Title"="&Congressional C.C."

[HKEY_CURRENT_USER\SOFTWARE\Access\Links LS 1998\Options\Resorts]
"99"="&Other Resorts"
"Logo"="restlogo.flc"
"Shadow"="restshad.gif"
"Background"="restbkgd.gif"
"10"="&Kapalua Resort-Maui"
"Icon"="resorts.flc"
"IconLZ"="contour\\resorts.ls"
"Title"="R&esorts/Tours"
"40"="&Congressional C.C."

[HKEY_CURRENT_USER\SOFTWARE\Access\Links LS 1998\CRX\Course5]
"Install"="0"
"Path"="CONGRESS\\CONGRESS.CRX"
"Name"="Congressional Country Club"
"Version"="768"
"CDTitle"="Congressional Course CD"
"Tour"="&Congressional C.C."
"CDPath"="CONGRESS\\CONGRESS.CRX"

8. Save the reg file, then double-click it to add it to the Registry. (Windows will ask you to confirm, say yes.) Then start the game. The Congress Course should be available and it should properly link to the Congress CC resort. The new resort should also be available in the menu, and the videos should play. The recorded players should also be available, and the flyby videos should play in-game.

This same method should roughly work for other resorts: create a reg file similar to mine, but use the values from the LINKSLS.INF of your particular CD. I hope this helps!

(edit: in my original post, I wasn't able to make the flyby videos work. I have updated it now with proper instructions for working flybys!)

Ray2515: Thanks for info)