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If you just need to download Arnold or one of the DCC plugins (such as MtoA or HtoA), go to the download page at solidangle.com/arnold/download.
You do need to be registered on solidangle.com (note to existing customers: this is not the same as the Solid Angle account you already have; you need to register on the web site too).
As this is a frequently asked question we have provided both the question and answer in this thread.