PCManFM: Not authorized to perform operation

Note: I am still GNU/Linux newbie, so please correct any errors/mistakes in this message if you see any!
Trisquel is based on Debian, so the commands should be the same or similar.
All the commands below should be typed in the terminal, and "sudo" gives administrator's privilages to any command following it.

You can find more information at:
https://wiki.debian.org/NTFS

First we need to find which partition is windows installed on.
To list all partitions on your hard drive, open the terminal and type in:
sudo blkid
On my laptop, windows is on partition /dev/sda4, so that's the one I want to access.
To create a folder, which will be used to mount that partition (make it accessible in GNU/Linux), type in:
sudo mkdir /mnt/windows

Now you can check if the folder was created:
cd /
ls
cd /mnt
ls
Which should show, that an empty "windows" folder has been created inside the "mnt" folder.

Now we can mount the windows partition in that "windows" folder:
mount -t ntfs /dev/sda4 /mnt/windows
If you now go to that /mnt/windows folder using for example the file system menager on Trisquel's desktop, or the "cd" and "ls" commands in terminal, you shoud see all your windows folders, and be able to access them.

From your comment, banan it seems that the partition is mounted (I.E. you see it in the file manager) and when you click it you can't open it, right?
You need to open a terminal and type in "sudo nautilus", followed with your password when it prompts you.
Then, a new file manager window will pop up. Now, the parition should be somewhere in the /mnt/ folder (At the root directory), so navigate there, if I'm not mistaken. If you can't find it there it could be in /media/root/ or /media/(username)/

Let us first see if the partition really is mounted and with what options. Execute this command in a terminal and show us the output:
$ mount

sudo apt-get install policykit-1-gnome
and then logout

Hello, I've got exactly the same problem with a Dell latitude D-500 with Trisquel mini 7. I neither can install the missing localization files because of the same root problem. To be root I need to use a terminal. For example I can't launch synaptic by clicking on the icon, I need to type gksu synaptic in a terminal.

I succesfully mounted the windows partition with the above command, but the user will need to be able to mount it with the simple graphical tools.
Installing policy kit and logging out did nothing. Edit : it worked ! But I had to reboot instead of logging out. ;)