Wordpress - Plugins not showing in dashboard->plugins section

Download the plugin as a zip, extract and manually upload the folder that immediately parents all the .php files (i.e not a folder that has another folder as its child) into you plugin folder.

Sometimes dev's can zip things within an extra folder, which confuses wordpress.

Per the WordPress Codex on plugin development, on File Headers:

The top of your Plugin's main PHP file must contain a standard Plugin information header. This header lets WordPress recognize that your Plugin exists, add it to the Plugin management screen so it can be activated, load it, and run its functions; without the header, your Plugin will never be activated and will never run. Here is the header format:

<?php
/*
Plugin Name: Name Of The Plugin
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: A brief description of the Plugin.
Version: The Plugin's Version Number, e.g.: 1.0
Author: Name Of The Plugin Author
Author URI: http://URI_Of_The_Plugin_Author
License: A "Slug" license name e.g. GPL2
*/
?>

The minimum information WordPress needs to recognize your Plugin is the Plugin Name line.

If I had to guess, I'd say the problem is that the Plugin Name header is missing from the top of the plugin's main file. Not knowing what plugins you're using, this is the most complete answer I can give right now.

EDIT

I should add that in order for a file to be checked for headers, it must be a .php file either in the plugins directory or in a subdirectory of the plugins directory. So, for example, wp-content/plugins/plugin.php and wp-content/plugins/my-plugin/plugin.php would be valid, but wp-content/plugins/my-plugin/lib/file.php would not.