How can I add a new VAT rate?

The first thing you should ask yourself is if this custom rate is added because of the fact that you are sending an invoice to a customer that requires another VAT rate or if it is because the goods or services you are selling are requiring another VAT rate. If the latter is the case, you should add a VAT group. In all other cases you need to define a new VAT zone that can be used for the customers with the custom rate. To get a complete understanding of VAT zones and groups work, please read VAT explained.

After the VAT group or VAT zone is created, you need to create the actual VAT code. Here you give it a name, put in the percentage and give it a label that will show up on the invoices when this VAT is applied.

The final thing you need to do, is to create VAT mappings for the new rate. You need to add a mapping with the new VAT code for each combination of the VAT zone and VAT group that is going to have this new rate.
For more information on this, please read the VAT Mapping section in VAT explained.