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: