The type or namespace name 'DAL' could not be found

Question

hi every body
i am developing an ERP application in 4.0
Web Appliction "3tier"
i have taken 3 projects inside one .sln file
one is DAL and one is BAL and .aspx.cs file
when ever i am adding a new class file to the project
and build it and add reference to the BAL/DAL and runs the application
it says  .dll  not found (or)assembly not found


Please Help

Thank You