Create a link to a folder on the left panel of Nautilus file manager?

You can add Bookmarks. Just go to the desired directory and via Global Menu → Bookmarks → Add bookmark.

Or you can use simply Ctrl+D when you are in the desired directory.

For Ubuntu 13.04 or more updated versions, you can click the "gear" icon on top right and use "Add Bookmark"

For Ubuntu 17.10, you can create links to the sidebar in the bookmarks file. In the terminal, type

cd .config/gtk-3.0

Then open a file called bookmarks with your choice of text editor. I use vim, so I type

vim bookmarks

Add the link to the directory (folder), save, and do a restart or log off/login.

it's very easy:

1.Open your folder
2.from menu bar select bookmark
3.select bookmark this location