Which version of Evolution are you working with?
When the "Site - Manage Membership" permission is added to the "Moderators" role, users assigned to the "Moderators" role should be able to edit the user's moderation status in two ways by mousing over the menu on the user's post.
1) Simply select "Moderate User". This will change the user's status to require moderation, and the next time you show this menu, the option will read "Unmoderate user" so you can change it back again.
2) Select "Edit User" and you will be taken to the user edit page in the control panel. From there you can set the user's "Forum Post Moderation" settings as desired.
Please let me know if this is not what you are seeing, and also any related errors either on-screen or in the exception log.