I've used Flash Aid successfully, there are 2 or 3 issues that have to be overcome before it will install flash properly.
Did mine a while ago so can't reproduce it, but look in the terminal window that pops up when downloading and installing flash.
Look for errors about file or folder not existing, I think from memory that there is 1 you have to create in /usr/..... and 1 in /root/.mozilla
Just create the folder structure it complains is missing and re run flash aid, (you have to restart firefox everytime before you can run it again).
Hope this helps