OK, so it wasn't obvious from the Gavick website but looking at the template manager revealed that the current version of postnote up on the site is several versions newer than the one I had previously installed.
This new version appears to work just fine with MooTools upgrade.
However, I now have a new issue with the column sizing. It seems that in the latest version adjusting the percentages for column widths (left left2, etc.) does not change my layout.