Technically, you can't. As Xperia Play does not have PSN, it has PSM (Playstation Mobile)
EDIT: Looked on different websites. Found out that at first PSN did not allow people to login from play in other countries except for Japan. But now it has been fixed.
http://www.xperiablog.net/2011/11/24/playstation-store-on-xperia-play-now-working-for-non-japanese-accounts/
Just use the same account that you logged in from your PSP Go.
(But I don't think it would work. As I looked it up, and in the images I could only find "PSOne classics" and no PSP originals.)