The ContactAdded delegate was renamed to ContactAddedEventHandler to match .NET standards. You only need to write the name of the handler method though, so both of these are correct:
ManifoldPoint.ContactAdded += new ContactAddedEventHandler(ManifoldPoint_ContactAdded);
ManifoldPoint.ContactAdded += ManifoldPoint_ContactAdded;
Events in .NET typically don't return any values, because several methods could be hooked up to the event and return different values. This is different from C++ where a single method is set as the handler. Since Bullet doesn't currently use the bool return value, void is used instead. So your method should be void and not return any value.
I think BulletSharp only recently got to a point in development where such breaking changes could be considered a bad thing. Sorry about that anyway.