Thanks. All.
I tried to delete the previous installation. By unfortunately I rm some rpm installation, which I should use rpm -e. That caused me one trouble: when I tried to install nsxml, I was told no libxml2 was installed. But when I tried to install libxml2 (rpm), I was told it was installed. Finally I decided to reinstall my OS (red hat 7.0) and reinstall aolserver/postgresql and libxml2.
After reinstallation, I can install nsxml and call it from my page. Silly, yeah, a little bit. But it solved the problem.
Thank you all for help!
