The name of the tiddler doesn't make much of a difference in this case.
Tiddlers tagged as "systemConfig" are normally loaded in alphabetical
order, and those loaded last take precedence. So if your tiddler was
named "aaa", and some other plugin named "zzz" would alter the username,
the latter's changes would override your plugin.
> Error: SyntaxError: unterminated string literal
Sounds like you forgot to close the quotes or something.
Let's try this again:
1. Create a tiddler, give it any name, and tag it as "systemConfig".
2. Copy and paste the following into the tiddler's body:
config.options.txtUserName = "Digital Doctor";
3. Save changes and reload.
That should do it.
More info here:
http://www.tiddlywiki.org/wiki/Configuration_Options
HTH.
-- F.