Installing official Google Marketplace on a chinese Asus Eee Pad Transformer 101
Apparently there is no way to do it.

There are some alternative market places that can be used, but they don't solve the fact that tablets with the chinese firmware are rendered useless because there's no Google account support.

So the only thing you can do really is install this:
http://forum.xda-developers.com/showthread.php?t=1245892

Works like a charm :)

Oh and for the Prime, Asus officially supports it.