Custom Monitor Problem

I need to set a threshold for monitoring freespace in GB, but for each drive separately. I tried to setup a custom monitor using (.1.3.6.1.2.1.25.2.3.1.4.5(.1.3.6.1.2.1.25.2.3.1.5-.1.3.6.1.2.1.25.2.3.1.6))/(1073741824)
where .5 is the instance from the GB monitor that you guys provide. This should give me the free space in GB on the M: drive of the server that I'm trying to monitor. When I try to add that custom monitor, it says that it does it successfully but it never shows up under the device.

This is a huge problem, we're trying to monitor our exchange server and when the transaction logs fillup the drive it crashes the exchange server.

I also have a problem with WMI not monitoring the exchange server, everything just shows up as N/A. I have the correct pwd and snmp is enabled. It was working before, I have tried to delete and re-add to no avail.

Thanks,
Mark