Thanks for the efforts at helping. Thus far, you haven't told me anything I didn't already know / do. Here's some more information and details in hopes that someone will find something that I'm doing wrong...
I open Aquaria, and on the screen with Naija in the blue crystal with various buttons on rocks, I click on the purple rock that says "Mods".
This brings me to a screen with a big box that says "Template Mod" and has a close-up picture of Naija. I can click on it, which opens the editor and places naija on screen in a nondescript circular room. Or, I can click on the ornate right or left arrows from that screen with the "Template Mod" banner. Clicking on the arrows cycles through precisely four Mod banners. They read "Template Mod", "Test Map", "Aquaria Jukebox" and "Tutorial". Those four were already pre-loaded when I installed the software from the Humble Indy Bundle, I haven't done anything to load them.
I wanted to access the other mods that people have made. So, after exiting the Aquaria program, I right-clicked on it's icon, and chose "Show Package Contents" from the pop-up menu. This brought up a new finder window. That finder window has a number of folders, the first one is named "_mods", the second is named "Ambrosia", then it has aquaria.png, then there's a folder called "config", a folder called "Contents" etc.
I double-clicked on the "_mods" folder. Inside it were a folder named "aquariaeditortutorial", a folder named "aquariatemplate", a folder named "jukebox", and a folder named "guert_mod". Each was followed immediately by a correspondingly named .xml file. Since three of them have the same names as the mods my software came with, I'm assuming these correspond to them, with "guert_mod" being the one that comes up as "test map".
So I downloaded a number of mods from this forum, and from the Aquaria modding wiki. These came in a variety of compressed formats, .zips and .rars, etc. I extracted each of them just as I've extracted plenty of other files before for other programs. I copied them from my downloads folder into the "_mods" folder inside the Aquaria package.
So now my "_mods" folder has in it those initial four folders and .xml files, plus it also has a "AngryLi" folder and AngryLi.xml, "beautyOfAquaria" folder and beautyOfAquaria.xml, "big_energy_battle_121" folder and big_energy_battle_121.xml, "rpg_PrequelCampaign" folder and rpg_PrequelCampaign.xml, etc. They look just like the format and structure for the existing Mods that came with the software. Poking around in the folders, I see each contains a number of other folders, an "entitygroups.txt", a "mod-icon.png", a "mod-init.lua", etc, so there's really nothing missing from those folders that is present in all 4 mods that came with the software bundle. Everything looks to be in order, and seems like it should work.
But when I open up Aquaria, and click on the purple Mods rock, it only ever shows me those same four Mod banners to choose from, just "Template Mod", "Test Map", "Aquaria Jukebox" and "Tutorial". It never shows me any other mod banners or names, no matter how many times I click through with the right and left arrows.
I'm left with two questions off the top of my head:
1) Should I have put the mods somewhere other than inside the software package? I do see that at least one mod has a readme.txt that says "To install place the CostumeMod folder and CostumeMod.xml into your Aquaria _mods folder (ex: X:\Games\Aquaria\_mods)" That example doesn't match the placement of my aquaria "_mods" folder at all. The default installation just placed my Aquaria in the Applications folder, so I've assumed thus far that the example given in that readme was probably the default installation location for the PC version. Maybe it/I installed to wrong spot on my machine? I'm pretty sure I just did the default installation and nothing special, I can't imagine why I would have done anything unusual, but it's been several weeks so I don't know for certain.
2) Has anyone on this forum had similar experiences with the Mac version purchased via the Humble Indie Bundle? If others who got the same version of the software as me are having no trouble running mods, then maybe they could take a close look at what I wrote above to see if I'm doing anything wrong. They could also chime in that mods work for their mac with this version, so that I'd know for certain that the problem must lie with my computer or my files, and not with the Mac version in general, or the specific build included in the Humble Indie Bundle.