Hi sundaypretzel and welcome to the Forums!
You can edit your save file on a Mac just like they did on a PC as I did the same procedure to the Stardew save file on my Mac to change my Farm Type from Hilltop to Standard. The bit in the procedure about using an XML plugin looks like something done for visual clarity while doing the edit (which is nice but not necessary), instead all that's needed should be a simple Find and Replace like for my Farm Type edit.
Let me go through the procedure and test it out to make sure it works and write up instructions.
Edit: OK it worked on my Spring 1 Day 3 save file. The tl;dr steps are:
1. Find your save file and make a backup! Always the best practice.
2. Edit your SaveGameInfo file to add in the recipe
3. Edit your FarmName_8675309 file to add in the recipe
In detail:
Backup Save File
1. In the Finder, select Home from the Go menu
2. Select Go To Folder from the Go menu
3. Type in: .config and hit Return
4. Open StardewValley and then Saves
5. Copy the folder with FarmName_########## and Paste a copy of it elsewhere, like on the Desktop or in your Documents folder. Look inside both folders and make sure they are the same
Edit your SaveGameInfo file to add in the recipe
1. In the FarmName_########## folder in the Saves folder right-click the SaveGameInfo file, select Open With and then TextEdit
2. Search for craftingRecipes
FYI Notes (ignore if you want): the text is a giant block as this is an XML file and all the info is encased between a series of <> with no spaces. If you read across the lines you will see a repeating pattern of item, key, string, value with Wild Seeds (Sp), Stone Sign, Wood Sign and so on appearing regularly between them. We will be adding in an additional line of this repeating text for the Ancient Seeds.
3. Between the first <craftingrecipes> and the <item> following it insert:
<item><key><string>Ancient Seeds</string></key><value><int>1</int></value></item>
4. Review what you pasted in to make sure you didn't add an additional space before or after the leading <craftingrecipes> or the following <item>. It didn't for me but you never know!
5. Command-S to save the file and close it
Edit your FarmName_####### file to add in the recipe
This is the same procedure as above but to the FarmName_####### file however the file is larger so you must be patient with each step.
1. In the FarmName_########## folder in the Saves folder right-click the SaveGameInfo file and select Open With and then TextEdit. This will take some time as it's a much larger file.
2. Search for craftingRecipes
3. Between the first <craftingrecipes> and the <item> following it insert:
<item><key><string>Ancient Seeds</string></key><value><int>1</int></value></item>
4. Review what you pasted in to make sure you didn't add an additional space before or after the leading <craftingrecipes> or the following <item>.
5. Command-S to save the file and close it
Run Stardew, start up your Save and you should see the Ancient Seeds packet icon with the recipe in there!
Edited again for little bits of extra clarity.
