http://archiverapp.com/I believe if you download that program (it's made especially for Mac) you'll be able to open the zip/rar files that come with item downloads. Once you've done that, there are usually directions on where the files are supposed to go but just in case;
.mesh, .material, .png files included in these ITEM downloads go into FeralHeart>Media>Items
The item.cfg should go into the Main Feral Heart folder. In most cases, you can just replace it. Though if you're wanting to combine multiple item packs, you'll have to open these .cfg files with a word processing program equivalent to notepad. (I'm not sure if Mac users have this or not.)
As for maps, this will also help you open some files they come in.
.FHO files should go into Feral Heart > Media > Objects
.FHM should go into Feral Heart > Media > Terrains
.FHS/.FHW should go into Feral Heart > Media > Sky
.Mp3 files should go into music/sounds (Whichever the map maker dictates).
Hope this helped.