First you need to go into settings and somewhere in there there's an option to allow non-market apps to be installed. It will give you a virus warning when you switch it on - that's fine, ignore it (you can switch it back off again once you've installed it)
Then, go here (just visit this thread using the browser on the tablet and then click this link)
Firstly Flash on mobile/tablet devices isn't 100% the same as Flash on your laptop/desktop. In other words they are not 100% compatible. For simple stuff like watching videos then yes it's fine. But for games etc. often it's not.
What you will also quickly find (and I found this out the hard way too when my son wanted to play miniclip) is that even if you do manage to get a game to load, chances are you can't play it because the game was not designed for a touchscreen but for a keyboard and mouse. And you don't have one on the tablet! Even if you did bring up the onscreen keyboard, it sits on half the screen meaning you can't see the screen to play the game.
Wow! Thank you icapturethecastle - it worked! I downloaded the Puffin Web Browser for free and all the CBeebies games are working now. There is a little mouse on the screen which you can move around and the games are working fine.