I've tested that LN UI can open websites, FTP directories, But I don't know how to open the local folder.

for example: www.google.com or ftp://172.10.10.59, click these hyperlink, LN UI automatically opens a new interface.

I'd like to say: you can't just open local resources. Direct accessing local files isn't allowed.
I can think of two workarounds. You can access the file manager from TIV 2000 onwards. This will work on the server.
If you want to upload or download a file you can use Client.upload.file() and Client.download.file(). Those will allow the user to navigate to a directory.
http://www.baanboard.com/baanboard/showthread.php?p=209090#post209090
See programmers reference guide for reference.

Hello,

LN UI client functions
The following functions have to be used when the client runs in HTMLUI mode:

Single file functions:

client.upload.file
client.download.file
client.show.file
Multiple file functions:

client.prepare.download
client.add.download.file
client.start.download
client.upload.files
client.get.upload.filecount
client.get.upload.file
client.delete.upload.file.object
Assisting functions:

get.ui.mode()
tc.is.html.ui()
tc.is.thin.client()
client.get.media.type
When one of the LN UI client functions is used while NOT running in HTMLUI mode, the following message appear:

This function is not supported in the WebUI: <function>

LN UI detection
Two functions are available to detect whether a session should run in HTMLUI mode

get.ui.mode()
tc.is.html.ui()
Implement LN UI support
To Support LN UI, the developer has to:

Cleanup the UI of the session by hiding form fields used for entering a client file or a client folder. Note: also remove checks of these fields (check.input or DAL is.valid hook)
Implement an alternative path for accessing client files based on legitmate LN UI functions.
Restrictions to LN UI support:

Starting a client application without a file is no longer supported.
It strongly depends on the user (as the user is in control when LN UI) whether a client application is opened.