Here is a guide to install the Adobe Flashplayer 11 (the beta version) on Ubuntu 11.04. This guide will explain how to install it on a 32bit and on a 64bit system.
To install the flashplayer on a 64bit system open a terminal windows (press Ctrl+Alt+T) and copy+paste the following lines:
sudo add-apt-repository ppa:sevenmachines/flash
sudo apt-get update
sudo apt-get install flashplugin64-installer
When done you need to restart Firefox and the flashplayer is installed and working.
To install the flashplayer on a 32bit system, first download the file from this page and extract the content to your desktop. Then open your Home folder and press Ctrl+H to show the hidden folders. Open the .mozilla folder and check if the plugins folder exists (if not you need to create the folder). Now copy+paste the libflashplayer.so file from your desktop into the plugins folder and you are done. Now restart Firefox and the flashplayer is installed and working.