I just spent 30 minutes trying to find the answer to this on Connected Data's support website, and via Google, to no avail. I finally figured it out by accident. I'm putting this here in case someone else is looking too in hopes they'll find it.
If you're trying to mount your Transporter (I have the Sync, but this should be the same) as an SMB device but it's rejecting your login, don't use the email form of your Connected Data account. Use your username instead. That is, instead of firstname.lastname@example.org, use "gordonmeyer". (Also, of course, makes sure you've enabled SMB login in the device settings.) The instructions that appear in the settings are not at all clear about which form is required.
As an aside, don't be surprised to discover that only shared folders show up on your SMB-mounted drive. According to Connected Data, this is a feature not a bug. I disagree. But the way to get around the boneheaded decision is to create a shared folder that's not shared with anyone else. Yes, really. I did find that on the support site.