A few practical hints...

Read the section 3.2.4 of this page carefully, first.
Here, you see something like this:

{&DSET_LI, GPIBREAD, IB_Q_LOW, "\035", NULL, 0, 10, convertPositionReply, 0, 0, NULL, NULL, "\030"},
{&DSET_LO, GPIBWRITE, IB_Q_LOW, NULL, "\017%c", 10, 10, NULL, 0, 0, NULL, NULL, "\030"},

Notice something? For GPIBREAD, the command is on the fourth field, but for GPIBWRITE, the fourth field is "NULL". For GPIBWRITE, the command sending out here is "\017%c".

Let's think a bit about this "\017%c". Here, AB300 expects to receive the filter position as "%c". To be more precise, it expects to receive the position in a particular format. That's why the tutorial says "The filter position (1-6) can be converted to the required command byte with the printf %c format."

What does that mean? Well, you probably should consider what exactly needs to be sent in your Arduino exercise. So, let's think a bit about it.

In AB300, it takes "\017%c" and %c is really the position you are sending. \017 is the command, but it is in Octal notation.

In your case with the Arduino, you need to send either "onoff1" or "onoff0". So, the command is "onoff" part. This is, obviously, in normal ASCII character. The value associated with the commands are either "1" or "0". These are in normal ASCII. So, in the scheme of printf format, "%d" will work fine.

That should get you starting, I think...