The simplest way to add keyboard layout in xfce is to edit /etc/X11/xorg.conf – I didn’t manage to accomplish the task via settings manager > keyboard > layouts . Just add the following section (or make the existing one read like this):

Section «InputDevice»
Identifier «Keyboard0″
Driver «kbd»
Option «XkbModel» «pc105″
Option «XkbLayout» «us,ru»
Option «XkbOptions» «grp:switch,grp:alt_shift_toggle,grp_led:scroll»
EndSection

Obviously, I added russian layout, so replace ‘ru’ with whatever layout you whant. The next option sets the shortcut to switch between the layouts. Last, add layout switcher to a panel to monitor the active layout of window in focus.