How to use GApps lookup field

we just bought ZOHO Creator with GApps after evaluation

right now i just created a form with GApps lookup field

but it does not work (the dropdown field not showed)

also i try to add a Users dropdown and use script with

Users:ui.add(zoho.gapps.users());

but i just got following error message

Error details:

Error occured while on Load script.

Some preliminary set up is required for this task zoho.gapps.users. Please contact the workspace owner to do any actions on this application.

please let me know how to resolve the issue

thanks

regards, Matthew