Claudio,
The System.LoadLibrary command is correct. It's trying to find a library called libnsjava.so, and it's not succeding. How are your starting aolserver? You need to make sure that the LD_LIBRARY_PATH environment variable is set so that libnsjava.so is included in the library path prior to starting aolserver. Try starting the server in the foreground:
cd /usr/local/aolserver
export LD_LIBRARY_PATH=/usr/local/aolserver/bin:$LD_LIBRARY_PATH
bin/nsd -fkt nsd.tcl
