How did you attempt to install the Adobe Flash plugin?
The steps (for system-wide installation) are:
1. close the browser
2. unpack the installer if it is compressed (tar xvf installerpackagename)
3. run the installer as 'root': sudo ./installerpackagename
4. follow the instructions - accept license agreements and tell the installer the install path for your browser (usually something like /usr/lib/mozilla).
5. run the browser again
If you've done all that, then tell us why the thing isn't working - are there any error messages at all, do you get video but no audio, etc.
To see if the Mozilla browser recognizes that there is a plugin, just type in this URL:
You should see something like this somewhere on the page:
File name: libflashplayer.so
Shockwave Flash 9.0 r31