Bus to Magic Kingdom, then monorail to Poly would be the easiest, but you could also catch a bus or boat to the Poly from Magic Kingdom. If you catch the monorail, you will be on the correct floor. Just get off the monorail, enter the Poly, and turn left. You can't miss it.
I was there during free dining last year and they were definitely taking walk ups for dinner. I had an ADR, but they were seating walk ups immediately and having people with reservations wait (not long). It was about half empty when I ate there.
If you know you when you want to eat there I'd suggest making a reservation. But Kona has been a restaurant we've able been able to get into without a reservation even when others were booked. Along with Jiko, its our favorite WDW restaurant.