It looks like Adobe broke the data bindings window in the latest release. I don't have a solution for it. You would have to enter the code by hand outside of the data bindings window.
If you have trouble with the syntax it appears the data bindings window works when accessed directly on the page instead of from an opened interface so you could add the value to the page and then copy/paste it. Hopefully Adobe will fix this soon... otherwise we will have to create our own Bindings window and that will take some time.