To enable VAT/GST support in Moneydance, make sure that the "Use VAT/GST" option is enabled under Moneydance ->Preferences->General (or File->Preferences->General on a Windows or Linux computer). Once you have changed this preference, restart Moneydance.
Now you'll need set the VAT/GST rate for each category. You can do this when creating categories, or view and edit all VAT/GST rates through the Tools->VAT/GST Settings menu. This will display a table where you can enter the tax rates for each category, as well as the category to which the taxes will be assigned.
Once a category has been assigned a tax rate and tax category, any new transactions that are assigned that category will be split into two parts; the tax part, and the normal income/expense part. The tax part of the transaction will be assigned to the tax category. This splitting of transactions is subject to the "Apply VAT/GST" check box in the transaction editor remaining checked.
If you have a large number of transactions that have not been split you can use the Tools->VAT/GST Transactions menu to display all of the transactions to which VAT/GST may apply (according to your settings) so that you can quickly set the rate or tax amount for each one. This window will only display VAT/GST transactions for one account at a time, so you will need to repeat the process for each bank account, credit card, etc which has VAT spending.