How to enable contrib and non-free repositories in Debian

As root you need to edit /etc/apt/sources.lst

Then add contrib and non free at the end of each line that begins with deb and deb-src just like the example:
deb squeeze main contrib non-free
deb squeeze/updates main contrib non-free
Updating SNMP and Downloading All MIBS
To get your missing MIBs perform an apt-get update and follow the instructions below:
% apt-get install snmp-mibs-downloader
Assure that this line is commented out in /etc/snmp/snmp.conf
#mibs :