Your application likely has a Program.cs file or similar with statement like:
System.Windows.Forms.Application.Run ( new Form1() ) ;
You should be able to change it to something along the lines of:
if ( app is not registered ) System.Windows.Forms.Application.Run ( new registrationform() ) ;
if ( app is registered ) System.Windows.Forms.Application.Run ( new mainform() ) ;
The test for whether or not the app is registered will likely be implemented in the DAL.
Be sure to have the data access code in a separate class; not in the form.