This dialog could use an upgrade

You have to click each individual layout. No shift+select or anything. Maybe it’s due for a makeover for R9?

Rhino WIP has a new dialog for this command.
RH-50557 ImportLayout - Select All / Clear All
-wim

You can still right-click on an item in the list to isolate it. It helps when you need to select only a few items in the list.