Since you have created the section in Data-Portal class, the flow should also be created in the same class. If you want to use a flow that is present in another class, then you will have to add the reference in the Pages&Classes tab.
You can't refer like PageName.FlowName. All you have to do is Create an activity in Data-Portal class and in that create a new page with class of that flow which you are going to invoke. Then on this page use the method "Flow New". Example: Flow New TestFlow. Where "TestFlow" is a flow name.
I understood your issue. Basically you need to create a page referring to your flow action class.
First on the action set, add run activity action. This activity should be present in Data - portal class. Activity will have only one step, page new (give class same as flow act class).save it. Now create another action set launch local action, target - modal dialog. Using page-give the same page name as created in the activity.