I think the username / password is for your linked server when configuring the link. After you issue a request to the linked server, this username / password will be used. Make sure that the account you used to configure the link has access to the data source of your database.
You can also try using OPENDATASOURCE to access the remote server, although if I remember correctly, it is slower than using the linker server. It accepts a username / password parameter, so you must include it in your SELECT query.
MSDN contains several examples of how to use it.
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