Okay we need to check which Autocadd library you are using
DxfAttributeDefinition is used by WoutWare CadLib libraries
DxfAttribute is also included in WoutWare CadLib libraries
So I am going to guess it is that one.
When you ask these sorts of questions it is important you tell people what library package you are using, your question makes no sense without that detail.
Assuming I have the right library package a normal assignment process looks like this
DxfAttributeDefinition attribTag = new DxfAttributeDefinition('Tag', new Point3D(0, 0, 0), 5.0);
attribTag.Text = 'Tag';
attribTag.PromptString = 'Enter the tag';
attribTag.VerticalAlignment = TextVerticalAlignment.Middle;
attribTag.HorizontalAlignment = TextHorizontalAlignment.Center;
That comes straight from the WoutWare CadLib manual :-)