Dear andrewlong,
If you simply want to change the text without using a password, go into the “Medialab” folder on your computer and look for the “Styles” folder. Either find or create a text file named “substitute.txt. Open that file and copy the following message.
106,"Please inform the experimenter that you have completed this portion of the session", end of session
with whatever new text you want inserted between the quotation marks.
You can find the instructions in the Modify Message Text subsection of Section 8 Preferences and Options part of the MediaLab Manual (p. 81 of the pdf file).
Cheers!
Psychboiler