I suspect that this is a shunt capacitor, supplied to be fitted across the load (light) in order to provide the small amount of current needed to charge up the internal battery in the sensor. You say that you are not using a filament lamp. Fitting this across the light might well do what you need. However, I've never installed this sensor myself.
You will see in section 2 of the installation instructions:
"The minimum load must exceed 50 W. If not, please install the capacitor."
I strongly advise you to employ the services of an electrician for this job, particularly one who's installed one of these before.
