if you look in the example conf file at the bottom of that page, specifically at the wql settings, you should see how to query wmi. For example:
interval = 1
wql = select Name, DiskBytesPerSec, PercentDiskReadTime, PercentDiskWriteTime, PercentDiskTime from Win32_PerfFormattedData_PerfDisk_PhysicalDisk
disabled = 0
index = wmi_perfmon
If you need to know where in WMI to find the data you want, that's probably going to be all google. For example, the win32_computersystem class has a field for TotalPhysicalMemory. And the win32_physicalmemory class has a field called capacity.
I guess another option is to see if the add-on for windows would work for you too. Maybe that would be easier?