The functions/class you are using are part of Octave Forge instrument control package, they are not part of Octave core. While most of the packages there are compatible with Matlab syntax, this package is not. The syntax you are trying to use for Matlab only.
However, the instrument control package has quite good documentation with many examples on Octave's wiki. I would recommend you take a look there. Plus, you mention it's a USB dongle. Shouldn't you be using usbtmc
instead of serial
?