Edgar,

you need an xml Editor to edit the fixture types file.
1. Download JEdit, a free XML Editor
2. Go to programs, navigate to Catalyst Application
3. RIGHT Click on Catalyst Application and click "Show Package Contents"
4. FixtureTypes.xml is located in Folder Resources
5. Make a backup copy of FixtureTypes.xml somewhere else on your computer.
6. Open FixtureTypes.xml with JEdit.
You see in this File several other Fixtures already implemented. I would suggest You look out for a similar fixture and copy this, rename it and adjust Pixelsettings and what else is necessary.
7. When finished save File and then start Catalyst Application. If Catalyst crashes or won't start you did something wrong in the file, repeat to step 2.

Please note that this file will be loaded only once at startup of Catalyst.
That means to see changes you have made, you need to restart Catalyst after every edit of this file.

Good luck

Rolf