How to set "ecb-source-path" of ecb of Emacs?

This stupid question spent me a whole day, and I still don't know how to set it.

I'm new to emacs and ecb -- I installed it the first time today. The I found the "directory window" was empty when I started emacs:

So I read the document online, it says I should set ecb-source-path first, so I input:

M-x customize-variable <RET> ecb-source-path <RET>

Now, it opens a new document:

But I find I can't do anything to this document(can't modify it), and not find a way to set the path.

I tried again and again, googled a lot, still not lucky.

It really hurts me. It's too hard for newbies.

In the second screenshot is a button labeled "INS" if you click on it you generate a element of the customize list. Here you can enter your path.

Repeat for every path you want to add and don't forget to save with a click on "STATE".

Alternatively you can set it directly in your .emacs:

(setq ecb-source-path '("/i/am/path/one"
                        "/and/i/am/path/two"
                        "/there/is/no/path/three"))