When installing Zabbix on Debian 6, you may enconter an error with SNMP checks.
Error building asn.1 representation
First try to retrieve the most basic uptime value with snmpget
snmpget -v2c -c public 123.45.67.89 SNMPv2-MIB::sysUpTime.0
Cannot find module (SNMPv2-MIB): At line 0 in (none)
If you get similar output as above you are missing package snmp-mibs-downloader.
You can install this package via apt:
apt-get install snmp-mibs-downloader
After installation restart zabbix-server