The tool will never automatically withdraw funds from a 529 account. You must set up a manual one-time expense per transaction per account. The basic steps are below:
Go to My Plan > Expenses & Inflation
For each year you plan to use money from a 529 account, create a line item.
Select a 529 account
Select the amount to consume
Select the age you plan to incur the expense.
Select the reason
Repeat for each year you plan to make a withdrawal.