How to use the Path %XX% in BODS?

Hello,

I got a request to use the system path %XXX% for the flat file as the data target in the Data flow.

For example:

Set the variable $V_PATH_FOR_DATA_FILE = '%ETL_ROOT%' in the script, and then use the variable $V_PATH_FOR_DATA_FILE as the path for the target flat file.

However, when I ran the job, the path cannot be recognized as a valid path. But it is a valid path if I use it in the folder  browser.

So anyone has idea about how to define the variable for the system path? Thanks in advance.