The current delivered InDesign Host Adapter in CS Extension Builder 1.5 is only compatible with InDesign CS5 and 7.5.0. To support development for InDesign CS5.5 (including the 7.5.1 update) you will need to download the updated version located at:
Please note that this only affects Mac OS.
My reply was sent via email and got garbled.
It should have said:
<Host Name="IDSN" Version="[7.0,7.5]" />
<Host Name="IDSN" Version="7.0" />