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 1/4 to 1/2 of the specified mass of the driver.

I attach the mass to the center of the dust cap with 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 1/5 gram.

I added 2 ounces to the NHT1259 to get added mass results (the NHT is speced at a mass of about 4 ounces) and then went to the Driver properties dialog and selected "Use Added Mass" and entered 2 oz for the mass. For this test procedure, I get a free air result with the woofer pointing up, and then add mass and retest without moving the woofer. This gives best results (unless you can stick the mass to the woofer placed vertically which is really the right way to do it).

Unlike sealed box, which raises the resonant frequency, adding mass lowers the resonant frequency - with a subwoofer 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:

Here the red chart is the added mass curve and the black chart is the free air curve. 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.