Sealed Box and Added Mass are both ways to let Speaker Workshop approximate your driver's V_AS value. There is no doubt that Added Mass is easier and in some ways even more precise - the main issue with Added Mass is that it is more sensitive to temperature and humidity than Sealed Box.

To use the Added Mass method of testing a driver, just use a mass equal to about 1/2 to 1x of the specified mass of the driver.

I attach the mass to the center of the dust cap with a tiny bit of FunTak (a blue non-marking putty-like adhesive available at hobby stores and some drugstores and Home Depot stores). Weigh the mass and adhesive together for best accuracy. Since midrange/woofers can have mass in the 1 ounce area it is important to have this mass accurate to the nearest gram, if not 1/10 gram.

I added 1 ounce to the Vifa woofer to get added mass results (the Vifa is speced at a mass of about 17 grams, but I had a 1 ounce weight lying around) and then went to the Driver properties dialog and selected "Use Added Mass" and entered 1 oz for the mass.

Unlike sealed box, which raises the resonant frequency, adding mass lowers the resonant frequency - with a woofer that's an issue because the accuracy of the sound card is suspect at very low frequencies (and the jitter increases). Anyway, the following chart resulted:

Take a look at the subwoofer test for some more discussion of added mass testing. Once the added mass test is finished it's time to have Speaker Workshop estimate the T/S (thiele/small) parameters, including VAS.

That process is the same for both sealed and added mass techniques, so go here.