You need to escape quote characters with a backslash:
NSString *myJson = @"{\"author\":\"mehdi\",\"email\":\"email@hotmail.fr\",\"message\":\"Hello\"}";
Otherwise the compiler will think that your string literal ends right after the first {
.
The backslashes will not be present as characters in the resulting NSString. They are merely there as hints for the compiler and are removed from the actual string during compilation.
Newbie note: JSON strings that you read directly from a file via Objective C of course do not need any escaping! (JSON itself may need such, but that's about it. No need for additional escaping on the ObjC-side of it.)