Adobe Flash Player 10

The message is bogus.  They probably just wrote the error message text when Flash Player 10 was the current version. 

In general, what's going on is that the browsers are making it more difficult to run Flash Player.  The website is using JavaScript ot check to see if Flash Player is available, under the assumption that Flash Player would be immediately available if it were installed.  Again, totally valid assumption back in 2015, but it's no longer true.

Flash Player is not immediately available (because most of the browsers have implemented some kind of click-to-play strategy), so their JavaScript check fails and operating under this historical assumption, they tell you to go download Flash Player (which I'm assuming you've already done). 

You may need to play around with the available browsers to figure out what configuration works best for you (and the math site needs to start migrating to HTML5/JavaScript-only content).

You'll also probably need to do some stuff to enable Flash Player in your browser. 

Here's a guide:

https://helpx.adobe.com/flash-player.html

Also, if you don't have Flash Player installed already, you can get it here:

https://get.adobe.com/flashplayer/