Select the computer you would like to see (Hold shift and click to select multiple computers) so that they are all highlighted like the below

Then click "lock" on the right menu (Under the device sub heading). By default locked computers will say "Attention please!". 

To change this message click the arrow next to lock and select "lock messages"

Type what you would like it to say in the box and then enter a name in the "Menu text" field and click "Save". You can also tick "Make this my default" to default to this in the future.

To apply a non-default custom lock message click the arrow next to lock and then the name of the lock message

To unlock a computer select the computer(s) and then "Unlock" on the right Under the "Device" sub heading