For some courses on Labster.com, you are required to make a payment to access the simulations.
In this case, you need a course to enroll yourself in the course which will give you access to the simulations during the add/drop (grace) period for free. This period might be up to 10 days based on your instructor's request and you will see the remaining days on the screen.
During this add/drop period, you will be able to access the simulations for free by clicking "Continue" after launching a simulation within your course. After this period ends, you will be prompted to make a payment. Based on the payment options requested, you will either see 2 options (card payment and bookstore voucher) or just 1 option (bookstore voucher).
Here is how under 1 minute