For real-time streaming data buckets, your account access key gives you the right to stream data into your account. It does not give you access to read data from your account (your username/password authentication allows data to be read from your account).
You can find the account access key used to stream into a specific bucket by clicking on the "settings" link underneath the bucket name on the shelf. The bucket key plus your access key allows you to append to an existing data bucket.
*Note: For imported files or Tektronix Scopes with Initial State Direct Connect functionality, there is no access key associated with a data bucket.