It's not possible to create a new folder in Custom List in a Meeting Workspace through SharePoint UI. The only way you can create is using SharePoint PowerShell script.
spAssignment = Start-SPAssignment
$mylist = (Get-SPWeb -identity http://sp/mysites/meetingsite -AssignmentCollection $spAssignment).Lists["Demo"]

$folder = $mylist.AddItem("", [Microsoft.SharePoint.SPFileSystemObjectType]::Folder)

$folder["Title"] = "DemoFolder"

$folder.Update();

Stop-SPAssignment $spAssignment
Happy Share Pointing :-)