I think, you don't need dmdext if you are using a pinDMD v2.
You have to use the PinDMD2Slave_XP_compatible.zip.
See Page 2 in this thread for the download link.
Extract this File, copy the PinDMD2Slave.exe (32 or 64 bit) in your Pro Pinball Directory and start ProPinball.exe with the option "m3 dPinDMD2Slave"