In order to unlock the ability to craft a new invention at the Manor, you have to collect a set of Ben Franklin's almanac pages that are scattered around Boston and New York. There are four pages in each set of almanac pages, so it shouldn't be too difficult to do that.
Once you have unlocked an invention recipe, head back to the Manor (or any store in Boston, New York or the Frontier) and open up the crafting screen in the Accounting Book, which is to your left as you enter the Manor.
If you scroll down you'll see the invention that you have unlocked, and you must then acquire the ingredients necessary to craft it.
If you're unable to craft the required ingredients then it more than likely means that you need to progress further in Homestead missions to increase the level of the artisans.