When DMP starts, the GUI to connect is blank, nothing on it.

This means you have probably installed it in the wrong location. There should only be a single GameData folder. For example:

[mainKSPfolder]\GameData\DarkMultiPlayer\Plugins\DarkMultiPlayer.dll

If this was the issue, it has been fixed in dev:
http://chrisand.no-ip.info/data/kmp/darkmultiplayer/darkmultiplayer26-installed-wrong.png

Closing, There's no updates of this bug and it's definitely caused by an incorrect install.