Summary of Steps:

1. Navigate to the Account page
2. In the Account Tax related list, Click the New Account Tax button.
3. For the Tax Type, Select Purchase or Sales & Purchase. 
4. For the Tax Rate, enter the name of the Tax Rate 'Product' or use the Search icon to search for and select the appropriate Tax Rate.
5. If multiple taxes apply to this account, Click the Save and New button. If just one tax rate applies to this account, click the Save button.