you have to tell terminal the full path. Adam only gave part of a path inside the game file, like cd /Users/%your_username%/Downloads/PumpkinMac.app/Contents/MacOS
Additionally, even after those steps, the next time I right click and 'opened' it only presented me with "move to trash" or "cancel". But the _next_ time I tried it, I could also select "open". Now I"m in.