How to: Add and Reference Custom Assemblies
When you add a reference to a custom assembly in a managed-code form template project, that assembly is included within the form template file (.xsn) when your project is compiled and published.
To avoid a conflict with how the InfoPath project system manages files that are added to the form template file, do not copy any custom assemblies that you want to reference into the top-level folder of a form template project. By default, this will be a path in the following format: <drive>:\Users\UserName\Documents\InfoPath Projects\ProjectName
If you do want to move custom assemblies that you reference to a location within the project folder, you must create a subfolder under the main project folder, and then copy and reference custom assemblies from that subfolder. However, be aware that creating a subfolder for referenced assemblies is not necessary. As long as a referenced assembly is not located within the project's top-level folder, the InfoPath project system will copy the assembly into the form template file (.xsn) when the project is compiled and published.
Reference a custom assembly from its default location
Open the form template project in Microsoft Visual Studio Tools for Applications.
On the Project menu, click Add Reference.
Click the Browse tab, locate and specify the assembly, and then click OK to add the reference.