I don't understand what you are asking, but did you try to connect receiver to n900 at all? On the other hand extmou has nothing to do with hen or USB, so if you have problems connecting USB devices please use some relevant thread, you may receive more help there.

To be honest I would recommend you to buy bluetooth mouse as for now USB hostmode charging does not work, so connecting USB mouse will give you large battery drainage. Not to mention the pile of cables and hub if you need keyboard attached too.