As far as I can see, there is no way to link to a bundle, you need to "load" it, effectively making all its symbols available.
i.e.
[[NSBundle bundleWithPath:@"/Applications/Xcode.app/Contents/Applications/Instruments.app/Contents/PlugIns/MemoryPlugin.bundle"] load];
Update
OS X ABI Mach-O File Format Reference
The MH_BUNDLE file type is the type typically used by code that you load at runtime (typically called bundles or plug-ins). By convention, the file name extension for this format is .bundle.