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The fact that you did not get the credentials challenge during the command-line operation could have several explanations.
Possibly, you did not use /usr/local/bin/svn, but rather just "svn" (which might be the Apple-supplied one). If so, then try again with /usr/local/bin/svn. That's important, as this is the one that's part of SCPlugin, and it uses the same code and logic as SCPlugin.
Or, possibly, your repository is configured to allow check-out without credentials. To see if this is the case, try a check in of some (innocuous) sort. If you get the challenge then, that's just as good as the original experiment: give the user name and password that works, and that should be remembered for SCPlugin as well.
Or, possibly, you have credentials cached already. To get around this, remove whichever file in ~/.subversion/auth/svn.simple/* refers to the server in question. (Or, you can just remove them all; they'll be rebuilt as you continue to do Subversion.)