If you pick an item from the inventory and want to get rid of it again, just use the right mouse button.
Rotating the sign also needs the right mouse button. Unfortunately, none of us has a Mac, but I remember Mac mouses/mice(?) having only one button back in the days (don't know, if that still applies today). May this be your problem with the game?
You defenitely need both buttons to play IMAGinE. Right mouse button is "Looking at things".