where to assign fields to group

maybe I miss something,
- added some new cck fields
- created some new groups
- Groups are defined to show up as tabs
- Display field is set to show up as tabs as well

I can see the tabs and all fields in view mode but selecting this tabs does not change the fields, so they do not belong to a tab.
But where to define that field1 belongs to tab1 can not see this options. first approach was to do by ordering in display fields.