make sure you are logged in then you will be able to access the newest lectures as shown or you can scroll down or open the right handed drawer to access older lectures.
open the category of your grade and navigate to the lecture you need
open the lecture, a message appears as following which contain the credit cost of the lecture and your balance
click at purchase and you will enter the lecture and can start
And if your balance is insufficient, other options will appear like coupons codes, direct payment or topping up your wallet.