in the flow:

<action-state id="create">
<evaluate expression="operatore_rer" result="flowScope.operatore_rer" />
<evaluate expression="mainService.getComuni()" result="flowScope.comuni_list" />
<transition to="nuovo-operatore-rer" />
</action-state>
evaluate expression doesn't call the method mainService.getComuni()

where and how i have to declare mainService????