MUI Builder works fine on MorphOS and is good to learn the basics. I would also read the autodocs, "libraries/mui.h" and check the example programs included in the SDK.
Lately I've finished adaptate MUI++ for MOS... unfortunately i must do same changes in api, but You can test it: http://kaczus.republika.pl/2b_MUIPlus_MOS.lha Docs is not finished yet...
because that I said the builder is useless... it's much better to code own properly code and learn the things alone than to modify (maybe bad) source from others.
Better spend some time in learning to code than to think some builder can help you. That's my opinion.
For same reason I also avoid WYSIWYG-HTML editors.