The object (named as specified using ClientObjectName property) should always be available. You can check the page source code to see if it's used properly (search for /* DayPilotPro: 5.8.1956.0 */ string to see the JavaScript generated by DayPilot).
Two notes about the Command event handler:
1. Response.Write doesn't work here, it's an AJAX callback and it would corrupt the output.
2. You must call Update after binding the data:
DayPilotScheduler1.Update()
This tells the control to refresh the data on the client side. Otherwise nothing happens.
You might also want to check the tutorials section for working VB.NET examples.