I refilled my tank last week after getting this message. Supposedly you're responsible for refills in between services but depends on your dealer. I got this 2.5 gal Peak DEF from Advance Auto parts:
It comes to about $11 after the 20% discount or w/e coupon they have going on.
It comes with a filler tube, but its too big to fit into the DEF refill hole on the 335d so a small funnel will be needed. As mentioned there are 2 holes, one for the main tank (lower hole) and one for the backup tank. I didn't have to use all 2.5 gallons to fill both tanks (I probably used about 2 gallons). After refilling, the message goes away on its own. There's no need to reset anything.