One review stated that "The flap in the middle had to be removed after installation as it did not open up when the dryer was in operation." Is it marked for top/bottom? Is the air direction marked?

Yes, the air direction is marked. Here is the link to our website where you will find the installation video. https://www.imperialgroup.ca/product/venting-flexible-duct/dryer-venting/accessories/dryer-draft-blocker?query=VT0106 - SC