Hmm... from what I can see you're doing it right. Probably not a build thing; maneuvering thrusters have been in the code since April or so and haven't changed much since then. Odd about the index -- my test modular tables doesn't even
have the index line and it works fine. Good thinking with the index check. Lemme dig into the code a bit...
Well it's a little confusing, but it may well be a bug. As far as I understand this area of the code, it looks like index is tested against ship->num_maneuvering, but I can't see where num_maneuvering gets set. So if you specify an index, num_maneuvering never goes above 0, but if you don't specify an index, it works as expected.
So I guess we need to drag WMCoolmon or somebody in here, somebody more familiar with this section than me anyway

We'll have narrowed the problem down for them at least. But for now I think you can skip the +Index line entirely.