@Paris
Not true, the free version lets you have a signature, it's just that the automatic promo signature ("Sent with AquaMail") will be added too.
@NCdecolover
Sounds like you opened the app's global settings (menu -> settings).
Please be sure to start by long pressing your account -- even if it's the only one.
Hope the screenshots (below) help.
PS - "AquaMail support" is yours truly... also "the developer" and also "the designer"