Another way to get to open the component designer in VS 2005 is to double click the business object from the Solutions Explorer. This will open the component designer. At this point, open the property sheet and click the button next to the RequireFields collection. Check the fields that require validation and click OK.Side Note
If the fields do not show up in the editor, it is because the project has not yet been built. Build the project and try again. This occurs since there is no valid Assembly to reflect off of to retrieve the field properties generated through the Business Object Mapper.