If your site has a mixture of PDR-6000 series and or PDR-3 or 5 series you will need to use CMX 3.2. - Download Link
When adding 3 series or 5 series DVR's they can only be added to the 'Main Group'. Additional groups can only be made from entire units and the only DVR's supported for groupings are 6000 series DVR's, PDR 2160, 3160 and the PDR 400IP.
If you are using a mixture of DVR's please do not use or upgrade to CMX 3.6 because this version of the software is not compatible with the PDR-6000 series DVR's ONLY the PDR-3 & 5 series.