Then I suppose I am confused on what you are asking.
If you're unfamiliar with VBA code, someone can help you step through the process. I'm going to guess that you've got copies of the functions themselves (fOSUserName() is a function you have to put in a global module, Now() comes ready-made in Access).
Let me try this again without the unintended venom.
Open a new module in Access. Paste in the code that you've got for fOSUserName() as a new function. (If you don't have it, look here:
http://www.access-programmers.co.uk/forums/showthread.php?s=&threadid=7799 ). Save the module (it doesn't matter what you call it; the function name is what's important.
Is the Comment field a memo that you want to be able to add to the bottom of? Or (as it sounds) is it a text field that can have more than one entry per record? It sounds like you have a continuous subform that records comments, and you want to have the username and date/time stored in additional fields. That's a little easier than adding to a memo field, so I'll hope for the best and go for that.
In your subform fields (the controls for displaying username and date/time), go to the Data tab of Properties and look for a property called Default Value.
For the date/time field, it should be =Now(). For the username field, it should be =fOSUserName() (or whatever you called your function). You can even make these fields invisible (Properties>Format>Visible: No), or at the least set Locked: Yes and/or Enabled: No so your users can't mess with your audit trail.
As for my initial response, all I can say is this:
Never attribute to malice that which can be adequately explained by stupidity. Never attribute to stupidity that which can be adequately explained by poor communication. Always assume communication failure, even if it looks unlikely. (Elasait’s corollary to Hanlon’s razor)