I don't know if you've read Gavin Menzies book, 1421? This discusses the theory that the Early Ming dynasty explored huge tracts of the world, before suddenly turning isolationist. Personally I thought the book was interesting, raising some interesting evidence, although the author stretched that evidence a little too far for my liking.
So assuming China did not become isolationist in the 15th Century, I don't see any reason why they would not get involved with the Colonial Land grab in the New World. Ming dynasty China was one of the big powers in Asia (and indeed the world), possessing far greater resources and man-power than any contemporary European country.
Obviously the Pacific ocean is a bigger ocean to cross than the Atlantic, so this will hamper Chinese colonial efforts, but I don't think a Chinese colony or two on the western coast of North America is implausible. Australia may also be a potential site for Chinese colonisation.
The really interesting question is whether or not a non-isolationist China can resist invasion by the Manchu's, who founded the Qing dynasty. If the Ming dynasty survives beyond the 17th Century thats going to create some interesting butterflies.