A. When .Net builds the library it always goes to a DLL, but it's a .Net Assembly DLL not a COM dll. Unless you actually need to use this .Net assembly from a COM application, you do not need to check "Register for COM Interop".
B. There is not startup object in a Class Library. It's a library, not an application. Sounds like you need to create another project (Windows Forms App perhaps?) so you can test your library.