To be honest it would say that the importer is a WP plugin so add and activate in the normal way. It will import groups/forums/topics/posts and users from the available source systems. It requires an installed but empty SP system and the source and target DBs on the same server.
Then it would say run it and read the help panel on the front page!
But…. I have never come across Ning so I am afraid there is no Ning importer. Depending on the complexity of this Ning it might not be too hard to create one though.

took a quick look at ning, and it doesnt even seem to be forum… looks more like social network (ie facebook)…  is that right?

Thanks both for such a speedy reply.
Yes, Ning is more of a social network, but it has a forum within it.
I know I can import the members to WordPress and hence Simple Press, but I’m really keen to import 2 of my sub-forums as well, as they have lots of content that would be useful to my future students.

An example of these forums is http://thetransformationclub.ning.com/group/28-day-meditation-challenge/forum

Ning also includes groups, which may (or may not!) import well into the Simple Press structure.

I know that you can do back-ups from Ning, so they must produce some sort of export file. Not being a techy, I have no idea how this data might be imported elsewhere.

Thank you for your help.

So as Andy says, currently no importer for Ning…   If you wanted to provide us a database of that forum including the users, we could take a look at it and see how hard it would be to add it to the importer…  cant make any promises but we can look..  if that works, you could email it to us at support at simple-press dot com