Is filecryptsb the name of the executable?  If so, you need to add this in the Executable field and the remainder of your command [--encrypt test.txt --user test] in the Arguments field.

Also, make sure that you don't have any file path issues.  Check to make sure that the executable is in your PATH environmental variable, or set the WorkingDirectory value to tell the task where to find this file.  Same thing with the source file - you may need to use absolute path to ensure success.

To modify your task to use a variable, you need to create the variable in the designer, and then you can reference the variable in the Expressions tab.  For example, I am using the Expressions page to modify the Argument value with the following:

"d:\\web\\cgi-bin\\awstats.pl -config=wnj -logfile=" + @[User::LocalFile]

hth

Tim