I've got the same problem due to having both Inno Setup 5 and 6 installed: IDP installer tweaked only first one.
The lines to be added are quiet different to what you tried. They must be like the following:
; Inno Download Plugin include path
#pragma include __INCLUDE__ + ";" + ReadReg(HKLM, "Software\Mitrich Software\Inno Download Plugin", "InstallDir")