snmp disk and memory problem

i want to buy the product but first i need to check the trial version .. all is working great but i have a problem with snmp on linux :

ManageEngine is installed on a slackware 10 machine and i have 250 computers in vpn and lan to monitor ... windows stations are well monitored but linux stations have problems ... snmp provides me only CPU utilization, description and network interfaces and no record in memory and disk utilization ... i configured snmd.conf as in ManageEngine help documentation .. also read this forum and no result ... on all my linux stations (23) it shows me no disk and memory utilization