Question: I've installed WordPress by the Software Centre, now what I'm interested in doing is adding plugins and the codex centre (<http://codex.wordpress.org/Managing_Plugins>) tells me that I should see a plugins option in the panel to the left-hand side of WordPress that, for some reason, I don't see. Here's my screenshot and here's what I'm told I'm meant to see (from this website). 

What I see
![What I see](https://i.stack.imgur.com/rdnO2.png)

What I'm meant to be [seeing](http://codex.wordpress.org/File:plugins.png).

Answer: 
Your `/etc/apt/sources.list.d/canonical_partner.list` file is corrupted not `/etc/apt/sources.list`. So try to replace the contents in that file by running the below command on terminal,

```
var=$(lsb_release -c | awk '{print $2}'); sudo sed -ri "1s:^(.*)$:deb http\:\/\/archive.canonical.com\/ $var partner:g" /etc/apt/sources.list.d/canonical_partner.list

```

It replaces the corrupted line with the accurate line for your Ubuntu release.

And finally don't forget to update the repositories by running `sudo apt-get update` command on terminal.
I just had the same problem.
The above command did not work at all. But upon inspection of the sources.list file, I saw a line that didn't belong there. I remember trying to add a custom package path. (it didn't work btw) This manual adding of a location was done within the package manager itself. This method cant be used again since the package manager doesn't launch any more because of the line I added. Trying to remove the line using text editor did not work either. The file would only open in read only mode.

Run terminal as root or super user.
(YOU ARE ABOUT TO USE A VERY POWERFUL TOOL IN SUPERUSER MODE. BE VERY CAREFUL)
type:

vi /etc/apt/sources.list

This should open a text editor within the terminal

use arrow keys to navigate to the invalid line.

ALL VALID LINED SHOULD START WITH DEB

on your keyboard tap dd while the cursor is in the invalid line (the d key x2)

NOWHERE ELSE

If the line is successfully removed, hold down shift and tap zz (the z key x2)

Rather exit the terminal and call a professional if something goes wrong.