for macs:

1. click the link for the addon
2. open zipfile and see single inner folder, do not drill in (* folder will have the same name as the addon)
3. drag single folder onto your desktop (* remember where it is)
4. open applications folder
5. drill down into games / world of warcraft / interface / addons
6. you should see all other addon folders
7. drag the addon folder from #3 above into the addon folder
8. you should now see the addon folder next to all the other addon folders
9. restart your wow

Please note:

When you press download, the website "serves" you the file, your browser may then give you the option to open or save the file. Im guessing you are pressing Open, or your browser (probably safari) is configured to open the file. If given the option you need to "save as" this will save the .zip file. When you open the file in your browser, it "ruins" the file as safari, or your browser doesn't know how to open the zip file.