I recently bought aero bars for my road bike and it has an Issimo Nob on one of the bars. My questions is that I do have a computer installed on my bike (cateye) but is it possible to assemble that on the Issimo Nob? I literally have no clue how to assemble it on there because it would seems to me that it would be tilted the wrong way if I were to strap it on there.
Any ideas?

How is the nob mounted? Usually those stick off of the aerobar extensionssuch in a way that it is parallel to the base bar. You would mount it on that as though it were mounted on the bars of your road bike. Unless your cateye is mounted on your stem.

It's actually mounted on the stem, would I need a different setup to be able to use it on the Issimo?

You could probably mount your cateye on the stem of your aerobars. Failing that, the nob can be mounted somewhere else tha is perpendicular to it's current orientaion, or maybe the cateye can be mounted on bars too? The nob is basically intended to simulate the bar for mounting purposes.