Hi and Welcome,
It's highly recommend you read through/watch the tutorials on site
before posting IMVU questions as the basics are covered in those. Making content for IMVU (any game in fact) requires you first learn how to use Blender before then familiarising yourself with the export process for IMVU.
Understanding 3D involves a number of concepts that are to grasp if you're new to, it so it's best to take the time to read-up so as to save/limit the amount of confusion. If you have any questions afterwards you'll be better able to frame them within the context of what you've learned and what you're trying to do.
In answer to your question though, technically it doesn't matter where you place the mesh in Blender because IMVU uses 'nodes' to correctly locate the avatar in relation to the mesh (see the above on reading/watching the relevant tutorial material for 'nodes').