Are you running the latest Joomla One-Click? Mine is 1.5.9 and the Legacy plugin can be turned on. Does yours have the red X icon in the Enabled column? And green check marks for other plugins? I don't have a single greyed out plugin in the entire list. How about you?
My installation is a standard Advanced One-Click with only one module added (Calendar). I don't use it so I've not modified the site, other than the calendar.
Here's my entry for Legacy:
Plugin Name Enabled Order Access Level Type File ID
1 System - Legacy Public system legacy 29