Here is a guide to install the Install 64-bit Adobe Flash Player on Ubuntu 11.04 64bit.
First download the 64-bit flash player
Before installing the 64-bit flash player you need to check if there already is a plugins folder in your .mozilla folder. Click on your Home Folder launcher and press Ctrl+H (this will make your hidden folders visible) and open the .mozilla folder and look for a folder called plugins. If the folder isn’t there you need to make it first, open a terminal window (press Ctrl+Alt+T) and copy+paste the following line:
sudo mkdir -p .mozilla/plugins
To unpack and move the downloaded file into your .mozilla/plugins folder copy+paste the following lines into your terminal window:
cd Downloads
tar xvzf flashplayer10_2_p3_64bit_linux_111710.tar.gz
sudo mv Downloads/libflashplayer.so .mozilla/plugins
And that’s about it, now (re)start your Firefox type: about:plugins to see if it’s enabled