You will need to select the 'Distribution Tab' on the Pass Designer and at the bottom of that page there is the option to 'Send Push'. Click this field to send a push out to your passes.
If you are still unable to send out a push make sure you have uploaded an Apple certificate to send out pushes to your passes. The pass certificates that PassKit provides are for demo purposes only and cannot be used to send pushes for your passes. Apple® requires that anyone who issues a Pass does so using their own certificate.
Click here for a guide on how to upload an Apple certificate.