A machine must be running to access its walkthrough. You are only able to access one walkthrough every 24 hours.

The walkthrough of a machine is available right after you have started the machine. You can find more information about a machine, including if it contains a walkthrough by hovering over the name of the machine.  

How to access walkthroughs when connected through Kali Linux

Click on the MORE menu at the top of your screen. When the walkthrough becomes available, you will see the Walkthrough menu option no longer greyed out. Before accessing the walkthrough you will be asked to confirm you agree with losing all the points associated with that machine.

How to access walkthroughs when connected through VPN

You can access the walkthrough by clicking on the three vertical dots (More Command) menu located on the far right of the lab machine row. When the walkthrough is no longer greyed out, it means the walkthrough is available for viewing.