ACL Permissions are access permissions. They are group-based. Go to Settings > System > Staff > Staff Groups, edit the group. You'll see a list of all ACL permissions, make sure that you have permission to what you need.
When you install a new plugin, usually your group is granted permissions to the plugin. However, if you have multiple groups, you may need to grant permissions to others who need access. By default, for security purposes, access is not automatically granted to all staff groups.