The documentation doesn't make this very clear, but merge is a method of certain types of objects in System Identification Toolbox, not a function that accepts numeric data. Type this command and it will show you the merge methods that are available to be called.
which -all merge
Note that the example on that page merges two models returned by the arx function, which returns an instance of one of the classes that have a merge method.
I recommend clicking on the "No" button at the bottom of that documentation page, next to the question "Was this topic helpful?", and indicate to the documentation staff that they should clarify the page to make it obvious that this is a method of certain types of objects, not a function that can be called with any data type. Alternately, if you've contacted Technical Support already as Star Strider suggested you can ask them to forward along that feedback. [Feedback tends to have more weight if it comes directly from a user.]
