The Audio Unit plugins NetFilter and NetSynth have been compiled under Xcode 3.2.6, to allow compatibility back to Mac OS X 10.5 (Leopard). They are "fat binaries", meaning they contain both 32-bit and 64-bit code.
NetFilter can only handle one configuration: stereo-in, stereo-out. If you try to use it on a Logic Pro channel strip which is mono, Logic will offer you the option to load it as "mono", but then it will fail and Logic will crash. This is exactly the kind of "dumb" limitation you can expect with such early-stage code, and I promise to fix it soon. In the meantime, the workaround is to convert the mono channel to stereo, by putting in an instance of Apple's Gain plugin, in "mono->stereo" mode. (Click on the insert button to add a new effect to the track, go to the Utility menu group, choose Gain and then mono->stereo.) The Gain plugin must come before NetFilter in the signal chain.
NetSynth also supports only one configuration: zero (audio) inputs, stereo output. Hence it's important to ensure that your VST chain in NetVSTHost is producing a stereo output. (This will be the case for most VSTi synthesizer plugins, so it's not a big issue.)