The first step is to make sure that you have the iCloud Drive turned on, on your Mac.

1. Click on the Apple menu icon in the upper left corner of your Mac.
2. Click on System Preferences.
3. Click on iCloud.
4. Tick the box for iCloud Drive to enable it.

Once you have enabled it, you can check to make sure it shows up on your Mac under Favourites.
Now you are ready to tell Astro Gold to use the iCloud Drive

1. Open Astro Gold.
2. It should communicate with the iCloud Drive, and you should then see Astro Gold showing as an item in iCloud.
3. Click on it and you should see your chart files to the right, and you should then be storing your Astro Gold for Mac charts on the iCloud Drive.