At first, I had no idea what you meant by a "thin plastic object (like a tongue)". The cover of the Logitech Harmony 890 is made of fairly soft plastic, so DON'T USE SOMETHING LIKE A SCREWDRIVER(jeweler's) to try to pry the two halves apart! You'll wind up with raised dings in the plastic. I wound up with a couple of tiny raised points by trying the above. Luckily I stopped trying this technique before I did any real damage.
My solution was a stroke of genius if I do say so myself. I grabbed a thin plastic guitar pick of mine, pushed it into one of the side tabs and presto....the two halves of the cover began to release...no broken tabs and no marring of the plastic whatsoever. Try it. You'll appreciate how effortless this technique is.