I created a class library with two business objects (OrderHeader and OrderDetail) so I can reuse in multiple projects.  I set the ParentRelationship field on the OrderDetail business object and built the project.

My Winform application references the business object library and I have dragged the two business object components onto the form. Now I trying to set the ParentBusinessObject property on the OrderDetail business object and I receive a message 'A parent relationship must first be setup before a parent business object can be selected.' 

Am I missing something?

No, you're probably not missing anything other than Visual Studio might be hanging on to an old assembly reference.  If you build your solution and then restart Visual Studio and you're still getting that message, make sure you didn't put the "parent" relationship on the wrong business object.

Sorry, my mistake.  When I created the business object library, I used the standard vb.net class library template instead of the StrataFrame Business Object Library template.  I recreated and it works fine now.

Ah, yes, there is a difference between the templates in the assemblies that are referenced by default.  I'm glad you got it working