Flash does not explicitly recognize the MouseEvent "On Key Release", but there is a reliable way to do it. It does have a slight delay in it's functionality, but works nonetheless.
First, create a variable text field using the text tool with the "abc" button depressed. Right click (Control click on the Mac) the text field and give it a variable name "keyread". Now we're ready to start. To begin with I created the following layers:
Set up a movie clip named Key Release Detector and place it on the stage. Give it an instance name "keyrelease". Edit the movie clip and add ten blank frames.
On the first frame set a Stop action, and label it "park".
On frame 2 set a Play action and label it "play".
On frame 9 set the action you want to perform on key release, in this case we set the variable "keyread" on the main stage to "Key is not pressed".
On frame 10 set an action Go to and stop frame label "park".
On the main timeline, create a button. Set the actions for the button as follows:
On (Key: Space)
Set Variable: "/:keyread" = "Key is pressed."
Begin Tell Target ("/keyrelease")
Go to and Play ("play")
End Tell Target
End On