Plugin to embed unity3d files – I can't work it out

Hi,
I’m trying to use this plugin to embed a .unity3d file :

http://unity3d.com/support/resources/assets/unity-wordpress-blog-plugin

I activated the plugin but if I try to upload the .unity3d file as ‘media’ , i get an error message

“WebPlayer.unity3d” has failed to upload due to an error
File type does not meet security guidelines. Try another…

this guy has it working –
http://www.atomk.com/

What am I doing wrong? Am I supposed to be trying to upload it as media? or just stick it on my ftp or something? I’ve read the plugin documentation; WordPress don’t seem to have any unity topics on the site/forums?

thanks, Ste