Yes, the interface has changed, but not all accounts are on the new interface yet (really weird, some accounts show it while others don't). We'll need to update documentation to include the new instructions (or both). Yes, when configuring the gateway it's always best to use the Merchant ID for that parameter. Sandbox recently had a massive overhaul so it's possible there are going to be sandbox server communication issues in the mean time until you've got it more stabilized.