You have to edit or add a .conf file, e.g. under unix:

mkdir -p ~/.nibble/;cat >~/.nibble/nibble.conf <server=1
rpcuser=nibblecoin
rpcpassword=YourPassword
rpcallowip=127.0.0.1
rpcport=18341
#addnode=machine...
EOF
chmod 600 ~/.nibble/nibble.conf;

This file is required, to setup a password and an user account. In that case the port is set to 18341.