Comment 13 for bug 1283558

Please just retry this command - Unity3D changes its checksum really often, and often we have a small delay until we notice such an update. When you ran it the checksum most likely wasn't updated yet, but at the moment everything seems up-to-date for me.

If it still doesn't work after updating I can also give you some instructions how to manually download & install the plugin, but often this is not necessary.