Each level is locked until you have completed at least seven full days since you started your current level. If you have done this, you just need to make sure you've looked for and clicked the "Level Complete" button on each level page. There is one toward the top right and another at the bottom right of each page.
It looks like this:
In this example, The checkmark on the right signifes that I have completed Getting Started and am currently on Level 1.
Since I haven't yet spent seven full days on Level 1, if I try to click Level Complete and move on early, I see this message:
NOTE: It is important to note that this is time-based, and not just seven sessions. If you begin on a Monday, the next level will be accessible the following Monday, and not on Sunday simply after you've completed the seventh session.
If you believe seven full days have passed, and you still see that message, please contact firstname.lastname@example.org, and we'll check it out for you!