Add button in Custom Tab in opportunity

Hi,

I have to create new tab in opportunity to allow item details .

I am creating New  button in the custom tab using list modification form as shown below.

How to create a List Modification Form?

Using list modifcation form the quick create screen is expected to come here.

When I followed these steps,

1) I din't get how to add fields from data model into the list modification form

2)I am getting dump when I click on New button in the tab when launching opportunity.

And the dump is not clear for what reason it is occurring.

Can you please help?

Thanks

Jayasri