The Devices tab lists all devices in the database.
Each device has a tick box associated with it.
Tick the appropriate box (or boxes) to apply the new activity to the desired device(s).
There are also four buttons on this tab: Enable all, Disable all, Filter (F3), and Show all.
Enable all - selects all the devices shown
Disable all - un-selects all the devices shown
Filter (F3) - opens the "Database Filter" window, so you can filter the display of devices
(the operation of this function is described more fully, here)
Show all - disables any active filters and shows all devices in the database