failed to install new package

Hi,all:

I've just install a Debian squeeze in the vmware workstation from a cd-image download from a mirror of the debian site. I have checked and edited the /etc/apt/sources.list file, That is:

##########begin------------

# deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official i386 CD Binary-1 20110205-17:27]/ squeeze main

deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official i386 CD Binary-1 20110205-17:27]/ squeeze main

deb http://ftp.debian.org/debian stable main contrib non-free

deb-src http://ftp.debian.org/debian stable main contrib non-free

deb http://security.debian.org/ squeeze/updates main

deb-src http://security.debian.org/ squeeze/updates main

# Line commented out by installer because it failed to verify:

#deb ://volatile.debian.org squeeze-updates main

# Line commented out by installer because it failed to verify:

#deb-src ://volatile.debian.org squeeze-updates main

#############end----------

but when I want to install the relative snmp package, the error is given:

root@localhost:/etc/apt# apt-get install snmp

Reading package lists... Done

Building dependency tree

Reading state information... Done

E: Unable to locate package snmp

Would you kindly tell me what shall I do next?