Okay.
You aren't supposed to be looking for
PHP Code:
eval("\$bbuserinfo[lastvisitdate] = \"".vbdate($dateformat,$bbuserinfo[lastvisit]).
", at ".vbdate($timeformat,$bbuserinfo[lastvisit])."\";");
You're supposed to be replacing the existing $bbuserinfo[lastvisitdate] lines with that line.
Replace your lines that you have with the above line.
And finally, if you're trying to insert ", at" into every instance where the date is called, this isn't the way to do it. The way to do it is through templates... But you probably know that since you've at least scanned through this thread to find an answer to your question.