You can try use the XPath API from the JDK and see what it takes to be able to evaluate your expression as you want it.
IMHO XPath can be tricky to get working, and there is very poor details what may be wrong. For example you cannot enable any TRACE logging from the JDK which provides details how it parses and evaluates.
It appears that the namespace for atom in your XML doesn't match the namespace you specify for atom in your previous post.
You could also try setting your xpath to /:entry/:title
That will match any namespace. It's a little sloppy, but may do the trick.