EasyCatalog is a plug-in for Adobe InDesign, and as such platform-specific features are handled by the InDesign application. Therefore, the extent of EasyCatalog compatibility with Snow Leopard is largely determined by InDesign.

EasyCatalog for InDesign CS4
Adobe have recently confirmed that the CS4 version of Creative Suite is compatible with OS X 10.6/Snow Leopard, so EasyCatalog should also run without issue.

EasyCatalog for InDesign CS3
InDesign CS3 is not officially supported by Adobe when running under Snow Leopard. Although this does not necessarily mean that InDesign CS3 (and EasyCatalog CS3) will not run, it is an indication that Adobe do not plan to resolve any Snow Leopard-specific problems in CS3.

EasyCatalog for InDesign CS3 is still a supported version, so we will endeavor to fix any problems as soon as they are discovered. Unfortunately, some issues may be with InDesign itself which are outside of our control.

Further information on Creative Suite compatibility with Snow Leopard can be found on the Adobe web site:
http://blogs.adobe.com/jnack/files/Adobe-SnowLeopard_FAQ.pdf