Business casual is probably the most appropriate for the event. Dress uniforms are the equivalent of a suit and tie, so dress as if you are going out to a nice lunch with someone in a suit and tie.
As far as warmth goes, try dressy jeans, slacks or a skirt (with leggings/heavy tights), a sweater layered over a mock-neck or long-sleeve top (for warmth - layers you can peel off one by one if it is too warm), and a winter coat. For shoes, warm boots (Great Lakes is the place Uggs was invented for) with a decent tread for traction. It's not a very long walk (2 blocks) so heels are okay if you're used to them.
If it's REALLY cold, you can wear tights or nylons under your jeans/slacks.
Don't forget a warm hat, scarf and gloves.
Once you're inside it's quite comfortable. You can use your coat as a seat pad on the hard benches.