The (fantastic!) new projectors in second and third grade classrooms try to install software the first time you connect your MacBook. It's okay to install it; it helps your MacBook connect to the projector.
Sadly, that software install lacks the drivers to make your whiteboard's interactive features fly. To make matters worse, it actually messes with and disables the click feature of your trackpad when you are connect to the projector via usb cable. Why does this happen in 2017? I don't know. Built in obstacles are annoying. In this case, though, an easy solution exists. Simply install the Easy Interactive Driver (which Epson should have bundled with the projector's own software install). The Mac version is available from Epson's site.