Yeah, I would want at least 2 lenses, something for close work (like the kit lens, I'm guessing 18-55mm) and a zoom around 70-200 or similar.
The longer zoom will be particularly useful to capture people without getting in their faces.
If you don't have another lens, you could hire one for the day/weekend.
I would also suggest a Stofen diffuser on that flash to so you can soften the flash if you need to aim it directly at the subject.
I did a corporate event a while ago and you need to know what the organiser wants from the photos (do they want group shots, people at tables, posed in groups or casual, any portraits of individuals, will there be awards/speeches/etc from a stage).
Take some time looking at photos from event photographers to see what sort of shots they take.
Make sure you take at least 2 memory cards and 2 sets of batteries for camera and flash.
Take lots and lots of photos, you can always sort them later, but if you miss something you can't capture it once it's gone.