So strange!
The plugin work well, but the execute command for cr2hdr doesn't works...
Could you uninstall the plugin, install this version and paste the output log (normally MLDualISO.txt into your home folder or documents folder). You can replace your username/name if you want but keep space if present.
I hope I'll solve your problem quickly!
Here is the information from the log file:
2014-03-28 03:52:02 +0000, DEBUG Export cr2hdr file: /Volumes/Photo Vault 1/Underwater Photos/2014 Photos/2014-03-27/ML/140327-092716-0001-dualiso.DNG
2014-03-28 03:52:02 +0000, DEBUG Export cr2hdr output: /Volumes/Photo Vault 1/Underwater Photos/2014 Photos/2014-03-27/ML/140327-092716-0001-dualiso.txt
2014-03-28 03:52:02 +0000, DEBUG Export xmp file: /Volumes/Photo Vault 1/Underwater Photos/2014 Photos/2014-03-27/ML/140327-092716-0001-dualiso.xmp
2014-03-28 03:52:02 +0000, DEBUG Plugin Path: /Lightroom Catalogs/Plugins/cr2hdr.lrplugin
2014-03-28 03:52:02 +0000, DEBUG Command for cr2hdr: exec "/Lightroom Catalogs/Plugins/cr2hdr.lrplugin/bin/cr2hdr" "/Volumes/Photo Vault 1/Underwater Photos/2014 Photos/2014-03-27/ML/140327-092716-0001-dualiso.cr2" > "/Volumes/Photo Vault 1/Underwater Photos/2014 Photos/2014-03-27/ML/140327-092716-0001-dualiso.txt"
2014-03-28 03:52:02 +0000, DEBUG Problem with export to cr2hdr
2014-03-28 03:52:02 +0000, DEBUG Error: 11