Here's one way of doing it: hash a passcode and store it as a custom attribute in the JAD file. In the MIDlet startup code, get the user to enter the passcode, hash it and compare to the JAD attribute's value. For a secure solution use MD5. And consider avoiding annoying the user by flagging a successful entry of the passcode in an RMS store (and check that flag on subsequent application starts). All of this adds a few KBs to MIDlet's JAR size.
As a suggestion, use numbers for a passcode - users understand the concept of a PIN.