I do not have any experience setting up the Subsonic server, but from what I know about servers, opening firewall ports, and static IPs I can tell you: those have nothing to do with installing Z-Subsonic. Z-Subsonic is a client that accesses up to 5 different Subsonic servers. To install the app, click the blue Purchase button in the top right of the screen. The App store will then download the files to your phone, and the phone will automatically finish the installation. Easy as pie.
Using the Z-Subsonic client is as easy as knowing the web address, user name, and password to a Subsonic server. After inputting those in the settings menu, you will have access to the entire servers music library.
Could not be easier.
This app has never crashed on me, but I have an iPhone 4. It is pretty lame that the iPhone 3G is almost unusable, and I wonder if that is causing the problem. Again, the issue may be unrelated to the app. If the 3G doesnt work with the app then maybe that should be addressed.