On the battlecode website, I can select players to scrimmage against, but there are no options for maps to play on in the dropdown.
Scrimmaging is not yet ready on the website, but will be in the next couple of days! In the meantime, you can scrimmage locally against your own bots using the "Run Match" feature.
UPDATE: Now running!
I see some maps in the scrimmage. Challenge SENT! It's a test bot only with no extra logic, don't get too excited
It claims to be running.
Scrims are running now!