For the feature to be set to be continuously on, it says to "flip the wall switch off for 3 seconds and then back on". I don't have a light switch for this light it is just always 'on'. Is there a way to turn it on for an evening in this case?

The HGX LED motion-sensor units, both the PIR and the MO, have an override function that will allow you to temporarily bypass the motion sensor for the night. To put the unit into this override mode, it has to be dark because the unit has to be enabled by the photocell; you will then toggle the light switch the unit is connected to quickly off and then back on, no more than 3 seconds. This will bypass the motion sensor, and the unit will be dusk to dawn for the rest of the night. To take the unit out of override mode, you can toggle your light switch off and back on quickly, or you can allow the unit to stay on for the night and cut itself off via the photocell in the morning. Once either of these things happens, the fixture will be reset into motion-sensing mode.