Ok so I did eventually find a solution. I'm gonna put a little tutorial for everybody having the same issue:
- Open the Terminal
(- If you didn't already do it, drop the app in Application (it's better like this anyway)
- Now just locate the file in Terminal :
cd /
cd Applications/Platwormer\ Mac.app/Contents/MacOS
(If you renamed the app, type the current name instead of "Platwormer\ Mac". Don't forget the .app, tho)
- Then, just type in
chmod +x Platwormer
- Should work now!
Btw, really great game!