Best answer as chosen by user helly: I could be wrong, but,
the instructions you typed in your post do not mention that the app has to be running.
It does require you to edit the plist file, which is really only a text type file, and then run the app.
Somewhere in that plist is the line "Require Admin True" (Words to that effect)
Just use TextEdit, which is the default to open .plist files, and replace True with False.
Now run the installer again.
I have to wonder though, why are you not an Admin on that machine?