To request access to one of the private folders, go to your User CP near the top-left of the screen, then select Group Memberships. The two private groups associated with the folders will be shown there, allowing you to request to join them (if you're not already a member).
For the Erotic Lit. group, you must specify your actual age in the request reason. You don't need to provide any reason for joining the Writers Workshop though.
Once one of the moderators approves your request, you will be able to see and access the relevant private folder.
If you are already a member, you can also choose to leave the group here as well.