I get caught trying to remember this a lot and there’s a really useful tutorial on this at the Net-SNMP website: Using and loading MIBS.
If you’re using Ubuntu, also consider checking the comments in
/etc/snmp/snmp.conf which (in 13.04) contains:
As the snmp packages come without MIB files due to license reasons, loading of MIBs is disabled by default. If you added the MIBs you can reenable loading them by commenting out the following line.
Also, run the following:
apt-get install snmp-mibs-downloader
which will download some basic MIBs as part of the installation.