Ok looking into this you can't use =ImportXML
to import JSON data, this is purely designed to use for XML and HTTP, which makes sense with the error it was throwing up.
From what I can see Google doesn't yet provide a =ImportJSON
method, but I have found a script here which does then include this exact function.
I have created an example on this spreadsheet here for you.
So it basically just used this function instead ...
=ImportJSON("https://www.bitstamp.net/api/ticker/", "/")
You can then probably play about with the path to find the exact bit you're after.