When I press the register serial button nothing happens.
If the registration dialog disappears, it's usually a fault of another installed product, which is "stealing" the focus for itself, constantly putting its dialog over everything else. This problems are usually caused by online flying products that have their own chat/text window and try to put it on top of other dialog (a very questionable behavior).
The registration dialog hasn't probably disappeared, it has simply being put behind the main simulator screen by those other products
You should be able to check if this is your problem, by pressing CTRL+TAB after seeing the registration dialog "disappear". This should allow you to put it back in front.
If still doesn't work, try to quit from the offending products before registering and/or put the sim into Windowed mode before registering.