Arianna
I just tested RESFEN 6 with a window record from WINDOW 7.8 and it worked without issue. In the screenshot below you can see how I have my preferences set.
The path for the "Use custom DOE-2 template file" is:
C:\Users\Public\LBNL\RESFEN6\data\RESFENTemplate6.dat
If you still don't get the simulation to run after changing these Preferences, email me the log file with the E+ errors in it, which should be in the RESFEN6 folder in Users\Public\LBNL