I haven't played with it enough to be able to tell you for sure, but my guess is that you'll have to create another user account.
The built-in guest account is supposed to be highly limited in nature. You don't want to let random people walk in and just do whatever they want to your computer. You give them limited access, which includes not allowing them to install 3rd party applications to run on IE. These are the types of files that often cause viruses, so it's a legitimate limitation.
See if an additional account will have the capability you want. You don't have to password protect the new one, so anyone could still use it, but they also wouldn't have all the advantages an admin would, so you'd have some added security as well.
If not, let us know and we'll keep trying to find a solution.
Well, in this case, Flash Player. I'm not sure exactly how you'd do it, but as with most applications, there's probably a way to enable it for all users.
You may want to look and see where the add-in is located in your administrative account. If there is a similar location that is not under a user profile, you can put it there and it will become a setting for all the profiles. I am thinking of something akin to making shortcuts available on all desktops, rather than just for the profile you are working with.
If I get more time later, I'll try to take a look and see if I can find how that would work so I can give you detailed instructions. In the meantime, you can look around and see if you can find it.