On your mobile phone:

Go to "Upcoming Events"

Click on "Buy Tickets Online"

Click in the "Search for an event" area

Under the green "Search" button at the bottom, enter the presale access code (See image below)

Click "Add Code"

Your event will then appear on your screen

On you desktop computer:

Go to "Upcoming Events"

Click on "Buy Tickets Online"

Near the top of the screen, enter the presale access code (see image below)

Click "Add Code"

Your event will then appear on your screen