Want to create fileformat txt file with dynamic file name

Within fileformat the filename, tried $G_Load_Type

and tried to give $G_Load_Type = KPOP_20121106Facid.txt using datestamp within filename.

When i ran the job i am getting the following error:

Data flow New_DataFlow4|Loader $G_Load_Type

Cannot open file <C:/Users/Johnson/Desktop/KPOP_20121106Facid.txt>. Check its path and permissions.

How can i create files with dynamic name based on datestamp of the day of jo execution?

Thanks a lot for the helpful info.