how to add a plugin? I want to add a 301 redirect plugin to a Craft CMS 2.6 website. I only have user permission as well. 

Please give me steps on how to add my redirects to the back-end of the site?
You need access to the server your Craft site is installed on to install the plugin code in the craft/plugins folder.  

In addition to that, you will need administrator access to the Craft Control Panel to access Settings->Plugins in order to install any plugins.

Most likely, you'll need to contact the administrator of your site or whomever developed/still maintains it to progress.