stored find criteria cannot evaluate calculations to determine what data to enter as search criteria.
Use this method:
Enter Find Mode  --> clear the pause check box, do not specify criteria
Set Field [Employee::USerName ; Get ( AccountName ) ]
Perform Find 
This is patterned after the scripted find examples that you can find here: Scripted Find Examples
Note: In FileMaker, UserNames and AccountNames are not the same thing. Usernames are defined in Preferences. Account names are defined in Manage | Security. Then FileMaker makes it even more confusing by entering the current UserName as the default Account Name when opening the file triggers the log in dialog to appear.
Thank you for pointing out the difference between a UserName and AccountName in filemaker. It made a difference in the Find.