I used to use Memory Mixer, so I'm familiar with the software - and I'm a Mac user too!
Marilyn gave good advice about downloading and putting the downloaded files into a folder. I think what you're asking though, is how to get them into the layout you're making in Memory Mixer. If you've downloaded a background paper, you can add it to the layout like you would a photograph. The software doesn't know the difference between a paper or a photo - they're both jpg files. I think you can also do that with an embellishment .png file, though I haven't tried it.
If you want to install the freebies into the software so they appear in your paper and embellishment drawers its a bit more complicated, but quite doable. You can access the software components using finder, following the paths below through the folders in its file system. First close the Memory Mixer program.
For an embellishment: finder>applications>MemoryMixer(the folder, not the icon that starts the program)>components>embellishments. There you'll find lots of folders with names like "circles" and "corners". Make a new folder here and label it something like Freebies or Misc. Drag your freebies in your new folder. You can add more files to that same folder over time as you download more.
For a free paper you'll do the same thing, but this time your path is: finder>applications>MemoryMixer>components>backgrounds>paper.
When you re-open the program, they should be there ready to add to a layout like any other paper or embellishment.
If you download a kit from ScrapGirls, you'll want to file it by designer and kit name. This is a bit more complicated. Before you do this, look carefully how they've done it with their pre-installed kits. In my (older) version, the paper and embellishments are filed separately. These are the paths through the folders and sub-folders.
For paper: components>backgrounds>paper>paper packs>designers>(designer name)>(kit name). Papers go inside the folder labeled with the kit name.
For embellishments: components>themes>designers>square>embellishments>(designer name)> (kit name)>objects. Embellishment files go in the objects folder.
This sounds a bit intimidating, but its really pretty easy once you do a little exploring and begin to understand the filing system. I found the easiest way to deal with the kits was to build the folders and subfolders on my desktop, then install them using the paths above. If you use a color label on your outside folder, you can easily see which ones you've added and which came with the program.
I'm sure you know that with Memory Mixer there are limitations to the program, so be careful as you shop through the boutique. You will not be able to use the layout templates, layer masks, brushes, actions, styles, or Scrap Simple products sold by ScrapGirls. (These limitations are why I eventually gave up on MM and started using Photo Shop Elements, but that's a totally different story...)
Hope this is helpful. If you need more info or a better explanation of something, you can sent me a private message through the forum. Good luck!