Completing Mitox's missions will indeed get you an invitation to Stormfist, but I wouldn't be a big hurry to get there. Go ahead and explore all of Rivertown and Verdistis first.
If you can't find the lost explosives there is a way Raze is always talking about to retreive lost items...I'll go look for it.
Ok I found it.
Click here for instructions on how to get back lost items.
Another edit: Even if none of this works out, there are other things you can do to get an invitation to Stormfist.
