Concerning the subject of modding tools for Mac that can be used for creating assets for Moviestorm, I'm afraid I'll be of little help. For my work I use Fragmotion, but it's only available for Win systems; and it's the same case for other popular and affordable software, such as Milkshape 3D. I think Blender is available for Mac too, but AFAIK the plugin for converting the output into Cal3D format (the "native" format used by Moviestorm) only works in old versions of the software.
So the only option left, again AFAIK is 3DS Max, but it's mighty expensive and, IIRC, a few months ago a fellow told us he had failed to make the Cal3D plugins for 3DS Max work. I don't know if this issue has been fixed by now.
Regarding the specifics of modding for Moviestorm, I'd gladly lend you a hand. For a start, I'd suggest you to check the tutorial Chris Ollie published in these forums long time ago, for it was the one that introduced me into MS modding. I know it's based on Milkshape as the modding tool used to create the animations, but it will give an idea of the basic concepts and the general process to follow for modding for MS.