First, MacSNMP does not allow you to set any of the MIB variables.
Second, you're correct, the manual does indeed explain that in order to send traps you need to set the various associated variables within MacSNMP. The point that is not very clear is that these variables can only be set from the SNMP console and not from within MacSNMP. In the second paragraph on page 25 you'll see that it says, "To copy these MIBs to your console and set the variables, refer to the administrators guide that came with your console."
Support Information Services