You are actually on Serenity Bay so you would have to take the tram over to SB and there is a check-in right there as you get off the tram. They will deliver wristbands to your room the night before so you will be set to go!
There is a call button in the cabana but there wasn't a drink menu. However, the concierge had a bar server come to our cabana with a menu, took our orders and came back with our drinks.
The cabanas are not far at all from the restrooms or Cookies. You can always have the golf cart come and get you but honestly, it would be quicker to walk!
We always pay cash because we never had the option to present our KTTW to the concierge for anything since the cabana is charged to the room. And don't leave it in the cabana...

we always just give it to him when we leave for the day.