I’ve gotten this question enough that I’m posting this to explain about Mac OS X Lion and my diglloydTools software.
One workaround is to run the command line (Terminal) version of DiskTester using 'sudo', which will prompt for a password and thus allow permission. Be sure to install the command line tools first (found in the diglloydTools folder once installed).
Start Terminal, then use sudo as shown in this example (using your own desired command):
diglloyd:~ lloyd$ sudo disktester recondition "Macintosh HD"
This technique works for most permissions errors (when Mac OS X denies access to a program, typically -5000 error, this is not specific to DiskTester).
You can use the Launch As Root program to launch the DiskTester GUI with permissions suitable for getting the test done. I’m definitely not a fan of giving anything root permission, but this gets the job done.