In the Package Explorer in Eclipse, do you see the library listed (possibly under "Referenced Libraries")? If not, try adding it again: select Project / Properties / Java Build Path / Libraries and add the JAR again with "Add External JARs...".
If you click open the library in the Package Explorer, you can see what packages are inside the JAR file. Is there a package net.n3.nanoxml, or does it have a different name? Maybe, if the name is different, the tutorial you're following is for an older version of the library you're using, and they've renamed the package to something else.