Nessus vulnerability scanner

I installed nessus 3.21 on my ubuntu box.i want to use only command mode scanning.so did try to scan the machine with following command

• nessus -q -x -T html localhost 7143 neminath  123456 scanlist.txt resultsfile.html

its saying unknown command nessus.but nessusd is running on my box on port 7143 which i could find out with

• ps -e |grep nessusd

if i navigate to the :/opt/nessus/bin directory and try this

•  ./nessus -q -x -T html localhost 7143 neminath  123456 scanlist.txt resultsfile.html

i ended up gettingnessus : Could not open a connection to localhost what might be the problem?any help?