Hi Junoli,
In order to access methods from BLL, you need to create object for the classes in BLL.
Like below
BLL objBLL = new BLL();
objBLL.YourBLLMethod_Name();
If your BLL contains any static classes then directly you can call the methods in those classes with class name.
Like below
BLL.YourBLLMethod_Name();
Happy coding.
Regards,
Manoj