After clicking on a button with a piece, it becomes chosen (you can see which piece is chosen right now in the right top icon), when spacebar is being hold and your cursor is near enough to already built pieces - you'll see white lines (which shows how your new potential piece will be connected to already built ones), after releasing spacebar the piece will be placed if you had enough coins to buy that piece.
Hope that helps!
Viewing post in The YOG's Tower jam comments
Thanks! Unfortunately I wasn’t able to get it to work. I am using a Mac; I’m not sure if that’s the problem. It responds to my clicking, but it doesn’t seem to actually select the item. Like, when I click the button, it highlights the item, but when I release the click, the item is no longer selected. No matter whether I’m holding space or not, I don’t seem to be able to interact with the white circles. Sorry, but the game looks great and interesting how many options there are for building.