Dear Huang Farong,
Please find a document that (hopefully) answers your questions: it shows examples of simulations and a link to download 2 simulations prepared for you.
Please note that you should use an up-dated DART version: you are using DART V998 whereas the present version on the DART web site is V1395!
If you have difficulties understanding the new modes of DART, I advise you to practice with the Work Package WP0, described in the DART User Manual.
Is everything clear now?
Note that if you are only intrested by mean reflectance values in several viewing directions, simulating images may not be the best solution.
Best regards
DART Team