Pre-sale tickets are only sold online. Please visit the relevant tour page, click on the pre-sale tab (make sure you are logged in as a Frontier Member) and check the information relating to the pre-sale there.
You will see your Pre-sale Password (which in many instances will be the same as your login password), the start and end times and a link to the ticketing page on which you can enter the password when the pre-sale commences to buy tickets.
Depending on the ticketing agency, you may need to also be a member of their website as well (eg. Ticketek/Ticketmaster/Moshtix).