If you have enough space on C:, the default choice of C:\Program Files (x86)\Addon Manager is the most sensible one (which is why, is default, we don't make installers doing stupid choices by default...).
If you don't have enough space on C:, choose the root of another drive, like D:, E:, etc. and it will install into an \Addon Manager folder under it.
In any case, don't install *inside* the sim.
