To login on a TrueBike, you'll need to enter your pincode. This pincode is seperate from your account password. If you've forgotten it, here's how to change it.
You can also disable the pincode requirement for a specific bike, find instructions for this here
To change your pincode, log in to the athletes.truekinetix.com website.
Choose your own 4-digit number in 'TrueBike Pin'
Don't forget to save your changes by clicking 'update' at the bottom of the page.