Have you put a space between addpath and the brackets? If so try removing it as Octave may not recognise it otherwise (I use MATLAB most of the time, can't remember if this works or not).

The error indicates that it can't find addpath. This may be because of the space, otherwise have you fiddled with the path already (and removed the standard ones)?

If so you could 'cd' to the directory containing addpath.m and run addpath() from there.

Simon