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.
Note: If you access a walkthrough, there will be a deduction of your points. However, if you have submitted all of the flags for that machine, you have a free access without any deduction.
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 grayed out, it means the walkthrough is available for viewing.