You need to do the following...
first make a shape to be your light and apply a unique material to it...call it IES emitter or something useful
next in the Maxwell Scene Manager using the materials tab assign an IES emitter material to this material you just created in SU.
You should see different previews of the various IES lights that came with the Maxwell plugin.
If you don't see any light try reversing the faces ...
if this doesn't work try increasing the power or wattage of the IES light...however this is just to see if it is working
Essentially Maxwell works just like a camera so for interiors lit by lights you need to increase your exposure which is done by changing the EV value in the Maxwell scene manager..
as you do this the FIRE render should update and allow you to tweak the settings until you get the light level you want