Today we disabled the "Data" page due to some bugs, but it will be back soonish in a bigger and better version. ;-)
To get your data, go to the "Projects" page:
Then locate your project and observation and click on its title to go to the Observation page:
On this page, go to the Status tab:
Under "Finished observations", chose the single observation, for which you want to download the data, and click on its ID:
This will take you to the Data page of this observation with the download button.