I tried to get this working, but got the same error you did about permissions. :(
The more I read about it, the more it sounds like it won't work without changes on Nintendo's side. I found this post on the 3DS subreddit that said they could get it working on Windows 7, but not on macOS or Windows 10. My only thought is that maybe the 3DS is using an older version of the SMB protocol that macOS and Windows 10 don't support, but Windows 7 and Windows 8.1 do support it.
Unfortunately your best option is to probably open the back of the case, manually removed the SD card and use it directly from your computer. I did this recently (bought a 32GB microSD to replace the 4GB that came with my system) and I didn't have any issues with it at all. The microSD is formatted as FAT32, which your Mac should have no problems reading or writing.