I am fairly certain it is not an issue with your operating system...
You probably either were not able to extract them, or did not place the files properly
In an item download you will either receive 1. a compressed file pack, or 2. multiple downloads for separate files. You need to make sure that the compressed file is decompressed into multiple files before you are able to place the files in their correct location. The reason for a compressed file is so that it does not take up a lot of space, and it is easy to transfer between computers because it is only one file until it is decompressed.
1.
If you received incompatible files (like .rar, or .zip) you will need a program similar to winzip (I'm not sure what the mac equivalent is) in order to decompress files. Once you are able to decompress the files, check out the instructions below for where to place the files. EDIT: check out this application if you need to decompress files:
http://www.macupdate.com/app/mac/7880/openup-x/2.
Otherwise, when you have .mesh, and .png files make sure that they are placed under feralheart/media/items . They not only have to be placed in the correct folder, but you also have to have the required code (specified by the person who provided the items, usually in a README file) which needs to be added to your items.cfg file under your feralheart folder.
I really hope you were able to understand that... Good luck.