To add VAT to your invoices you will need to enter your VAT number in your company settings.
From the home page of the website, click on the "Settings" tab. Then click "Company Settings".
On this page you will see a field called "VAT Number". Enter your VAT number here and click "Save".
Now VAT should be added automatically to all Invoices issued by your company.
If you need to charge for VAT, but have yet to receive your VAT number, you can activate VAT on your invoicing by simply entering a blank space where your VAT number should be in the above picture. Remember to add your correct VAT number when you do receive it.