The plugins generally arrive in different flavors for different browsers. If you inadvertently downloaded the plugins for a browser other than the one you are using, the installer wouldn't be able to find its plugin directory.
Try downloading & installing the plugins directly from the plugin's main homepage at the software house that produces it, making extra certain you have the right versions.
And you can look for yourself at the following common locations:
Internet Explorer --- C:\Program Files\Internet Explorer\Plugins
Mozilla --- C:\Documents and Settings\Username\ApplicationData\Mozilla\Plugins
Netscape --- C:\Netscape\Netscape Browser\Plugins
If you're sure you've got the right versions, and they still aren't installing correctly, try looking through Add/Remove Programs in the Control Panel, and try uninstalling whatever current versions are there, if any. Then try a Registry Cleaner in case orphan entries are causing trouble. I like CCleaner for this sort of thing --- http://www.download.com/CCleaner/300...-10315544.html
--- then download a fresh copy of a plugin & try running it's installer.
Best of luck - sorry for the delay in replying - been very busy
. . . Gary