You may run into the following issue when using a Mac.

Question:

When downloading .zip files on mac the .zip file automatically becomes unzipped. How do I install the WordPress plugin without the .zip file?

Answer:

Do the following so your zip files do not open automatically after you download them:

1) Go into Safari Preferences.
2) Page titled General.
3) Scroll down to the bottom.
4) Deselect the box with the text "Open safe files after downloading".
5) The plug-ins will now download as a .zip file which uploads seamlessly to WordPress.

The key is to upload the .zip file via the WordPress plugin uploader interface (not the unzipped folder)