This is not telling you that you don't own the computer on which the folder exists. It's telling you that you don't own the folder. This is a very different concept of ownership.
On your Ubuntu system, your user account doesn't automatically have the ability to perform any possible action--some administrative actions can only be performed by
root, the superuser. However, this user is not any real person and, in Ubuntu, you don't log in as this user. Instead, Ubuntu provides facilities like
gksu to perform actions as
You should check out the three links above. Once you've read that, if you still want to change the permissions on that folder, here's one way to do it:
Type in the command
gksu nautilus and press Enter.
Now you have a file browser window running as
root. You can change the permissions with this. Any program you open from this will run as
root too. You should use this carefully and sparingly.
Make sure to close this
root Nautilus window when you're done, so you don't accidentally use it for something that you don't need to be
root to do.
Before you change the permissions (and/or ownership) on this folder, you should be aware that doing this to system files in Ubuntu can have negative consequences: you can make your system insecure...or even stop your system from working.
If these files have the wrong ownership/permissions because you brought them over from another system and did so as
root (for example, if these are recovered documents), then go ahead.
But if these are systemwide configuration files, programs, or you're not sure what they are but you're trying to follow instructions that tell you to do something with them ...then you'll be much better off to ask specifically about them. You can edit your question or post a new question to ask if it would be safe for you to change ownership/permissions for this specific folder (provided that you can disclose what this folder is and what you are trying to accomplish).