That's not an error, it's the normal message that comes out when you make a major hardware change, and changing the mainboard surely is. There's no need to reinstall or uninstall anything, and there's no need to re-insert any keys, your Serial Number will always stay in your Windows registry, what you lost by changing the mainboard, is only the activation for that Serial Number.
After that message, you should also see a request "Do you want to activate the scenery on this PC ?", and of course you have to reply YES and, if you have an internet connection available, the scenery will reactivate with the new hardware configuration and the "Machine mismatch" message will go away.
This, of course, provided you still have available activations left, of the 6 you had originally. If you don't (which is quite unlikely), you will see a different message after you try reactivate, stating you have exceeded your activation, in this case you have to contact us via email with your Order Number or Serial Number, so we can add new activations to your order.
Note that, none of this would be needed, if you used the User-Controlled Deactivation BEFORE changing the mainboard, as explained here:http://www.fsdreamteam.com/forum/index.php?topic=4082.0
The User-controlled Deactivation will allow you to make hardware changes even if you exceeded all your activation, without having to contact support, but you need to do it when the scenery is still active, before doing any hardware changes.