Hi There,
This means that you do not have psychtoolbox installed. If you have a recent version of psychtoolbox this should be installed as part of the standalone, if you are running from a different IDE though you may want to install psychtoolbox.
As a side note that may be of help, I also have used tasks where we wanted to track a key release, this post might be of help Measuring key lift time relative to stimulus onset
Hope this helps,
Becca