Question is, how to add LS to the existing website - in an Iframe ??

There is no need for an iframe, simply include the lightstreamer.js file in your page and start using it as shown in our examples.

- I mean, the existing website is on port 80, and the LS server is at port 8080, perhaps even at another IP ?

Lightstreamer can be configured to listen on any port. You can put it on the same machine listening on a different port or on another machine listening to the same port. The only thing to keep in mind is that, in some companies , ports other than 80 and 443 may be blocked, so it is better to have both servers listening on port 80.

Next question is, how to make LS update a 2D array in the main form ?
Lightstreamer data model is made of Items and Fields; you can see them as rows and columns.
I suggest to start checking out the general concepts document file for an overview of Lightstreamer