Search for this problem in these forums... it should be fairly straight forward. Turn ON .Net services in HSphere for your domain. If you're using VS.Net, compile your application, copy *.aspx, web.config, global.asax, etc., and create a BIN directory off your root and put the project's DLL file there.
-Dave
After you do/check all steps that WineIsGood suggested, and still have problems, tell us what you get, e.g. what errors, etc. I'm by myself running an ASP.NET driven web site and have no problems. Although it took time for me to find out what to copy and how to my web site at the beginning