The problem here is that starting with 10.6, the default compiler mode in OS/X is now to produce 64-bit rather than 32-bit binaries.
All of the publicly distributed Vamp plugins and hosts (at least the ones that we know about and refer to on the Vamp plugins website) are 32-bit only. But when you compile your own host and your own plugins on 10.6 using the default compiler flags, the results are 64-bit. Although OS/X 10.5 and 10.6 will happily run either 32- or 64-bit binaries, you can't load a 64-bit plugin into a 32-bit host or vice versa. Consequently the publicly available plugins will not work in your host and the publicly available hosts will not load your plugin.
The short-term fix is to compile your host in 32-bit mode, and if you are compiling plugins also, to do so either in 32-bit mode or as 32- and 64-bit universal binaries (or even 32-bit and 64-bit Intel plus 32-bit PPC).
To do this, use the flags "-arch i386" when compiling and linking a host; and either "-arch i386", "-arch i386 -arch x86_64", or "-arch i386 -arch x86_64 -arch ppc" when compiling and linking a plugin.
(The long-term solution is for us first to ensure that all plugins being distributed are compiled as 32- and 64-bit universal binaries, and then to do the same for the hosts. But until the plugins all support 64-bit architectures,
the hosts must remain 32-bit only because otherwise OS/X will run them in 64-bit mode and they'll be unable to load 32-bit plugins.)
This change caught us somewhat by surprise (stupidly perhaps) and although work is now under way to improve the way the default SDK handles and documents this situation, this work is not quite done yet. I'm sorry about all the inconvenience.