I am not exactly clear on what you are trying to do, but I believe I can shed some light on your situation.
ASP.NET is a web development framework built upon Microsofts CLR (Common Language Runtime). This allows one to develop asp.net applications in any .NET languages. The most popular are Visual Basic (VB.NET) and C#(CSharp.NET).
Any application built with ASP.NET will need to be hosted on a machine running IIS (Microsoft's web server). Yazd forum, on the other hand, is a java application, and as such needs to be ran on the Apache Tomcat server.
The easy answer is that these 2 technologies do not place nice together, and as such you cannot use them on the same site. Is there a reason you can't build you web application in JSP, so that it will be in the same language as Yazd forum? Or alternatively, can you use a .NET forum? (I can recommend yetanotherforum.net)
If you absolutely must use both technologies, you still have some options. There may be 3rd party extensions to allow .NET applications to run on Tomcat, or JSP applications to be ran on IIS. However, I have no experience with this, and would worry about the extensions being buggy and not feature-complete.
Another option would be to run one application on a subdomain, which points at a completely separate server. For example, you could have 'domain.com' be your main .NET application pointed at your IIS server, but have 'forums.domain.com' point to a completely different server running tomcat.
Let me know if that helps, and what solution you end up using. I will be happy to help in any way I can.