The builders block is an interesting tool and is very well thought out, to use it you need to place it in your world and yes it does build the structure for you however... you need to place an NPC (or 2) near it and give them the builder job, once you have done that enter the builder's block GUI using the NPC wand and choose your Structure, click on preview and you will see what the building looks like and if you want it to face another way turn off the preview, change it's orientation then turn the preview back on, once you are happy with its placement turn off the preview and select enabled then bob's your uncle the NPC's start to build it for you, after a while you have a brand-spanking new building and can build large villages in a matter of minutes (or hours depending on how many buildings you want)
regards V497_Vesper