This really is no different from when you create an audience with regular old NTLM (Windows Authentication).  The difference is that while the AD provider is set up by default in all environments, the extra membership provider (that you use for Forms Authentication) isn't included anywhere except in the web application where you install it.  To be able to find your FBA users in the audience creation tool, you'll need to add the extra membership provider(s) to the web.config for your SSP site in IIS.  At that point, the People Picker should start recognizing your Forms Auth users, and you can create your audience as needed.