Open the project you want to burn and press the burn icon in the top right corner.

This will open the burn dialog for the menu, you can edit your burn project menu here, change background, segement open frames add text and what not...

Once you are dine with your editing, you can press back to the preview and then use the big burn icon to the right of the preview monitor. This will open the following dialog where you can choose what disc type you want to use.

From there, you click on Blu-ray and follow the on screen instructions.