webfoot,
If your operating system is running in 32-bit mode, install the 32-bit MySQL package, otherwise you can install either (32 or 64 bit.) A quick way to determine if you're using a 32-bit Mac OS X or not is explained in
http://osxdaily.com/2009/09/07/how-to-tell-if-youre-running-the-32-bit-or-64-bit-kernel-in-mac-os-x-snow-leopard/ .
There are also no ports to open and nothing to configure in Abyss Web Server. MySQL runs on its side and PHP scripts you'll install will have to be configured to use the local database server (which they will usually detect with no issues.)