The extension is called Multiselect for Trello (it does not exist as a power-up).
The Chrome store acknowledges I've already paid for this extension.
The extension shows up in the browser as an enabled extension.
Thought reloads of he Trello board page would solve the problem, but no luck.
Tried toggling the extension on and off in conjunction with board page reloads to no discernible effect.
Any solutions are greatly appreciated.