OK, so you want to install a DLL file, but you have tried and it doesn't work.
Well, maybe your a Windows 7 64 User ?
Normally you would write:
Regsvr32.exe "C:\Program Files\pling\plong.dll"
In Windows 7 x64 you write:
C:\Windows\SysWow64\Regsvr32.exe "C:\Program Files\pling\plong.dll"