64 bit Flash in Ubuntu is still a flakey beta (as of this blog post). It can be downloaded from adobe labs. Once downloaded, extract it from the tar.gz - use archive manager or

m@mbx:~$ tar -zxvf flashplayer10_2_p3_64bit_linux_111710.tar.gz

tar will then notify it has dumped
libflashplayer.so
into our current directory then:

m@mbx:~$ sudo mv libflashplayer.so /usr/lib/firefox-addons/plugins

and flash should now work for all users and Chrome, if you only want it for individual users extract to:

m@mbx:~$ sudo mv libflashplayer.so /home/username/.mozilla/plugins