Ok, I just installed it in my MAMP installation.
1. Unzip the archive.
2. Navigate into the resulting folder to its assets/snippets folder and copy/paste or drag the maxigallery folder to my MODx installation’s assets/snippets folder.
3. Navigate back out to the MaxiGallery folder and open the maxigallery.txt file, then copy/paste its content into a new snippet in the Manager.
4. Create the assets/galleries folder.
5. Put [!MaxiGallery!] in a document’s content.
And I have the button to Manage Pictures on that page.
I think you copied the whole unarchived folder with its assets/snippets folder to your own assets/snippets folder.
The confusion here is because on a Mac, if you drag or copy the assets folder from the archive into the MODx installation’s root in Finder, it replaces the existing assets folder, so you have to go into it and only take the folders you want and put them exactly where you want them. If you’re uploading with FTP, it only adds what isn’t already there, or overwrites older versions with newer versions of what is there. It leaves everything else alone.
Even if you use a terminal command line, cd into the unarchived MaxiGallery folder, and
cp -R assets /Applications/MAMP/htdocs/modx
it nicely copies just what you want and doesn’t overwrite everything else.