LucisArt does work on an Intel Mac in Photoshop CS3 if you run CS3 in Rosetta. I have not seen any problems with my customers. You do not need to run Rosetta if you are using an earlier version of Photoshop.
Typically the error message you are seeing comes up if Photoshop is not being run under Rosetta. There are instrcutions on how to start LucisArt in Rosetta on the LucisArt tech support page
http://www.lucisart.com/Technical_Support.htm if that helps.
Other than that I would make sure that you unstuffed the lucisart.2.1.sit file before copying it into the filters folder. the Macintosh demo LucisArt installation instructions are as follows:
MACINTOSH INSTALLATION INSTRUCTIONS
First download and decompress (unstuff) the demo version of LucisArt 2.1 for the Macintosh which is a .sit file. (You must have or install the free Macintosh stuffit expander available at
www.stuffit.com.) Double click on the lucisart2.1.sit file to unstuff it. When the lucisart2.1.sit file is unstuffed you will see the LucisArt plug-in on your desktop. The LucisArt plug-in will either look like a Photoshop plug-in or a piece of paper with one edge folded down. Install the plug-in by dragging the entire LucisArt plug-in into the "Filters" folder which under the "Plug-Ins" folder in Photoshop. Make sure that Photoshop is off or restart Photoshop after installing LucisArt. Then open Photoshop and open an 8-bit image. Lucis/LucisArt will appear in the filters pull down menu. When you process your first image you will be asked to enter the serial number. The serial number activates the demo software.
Barbara