PHP + MODx are case-sensitive. Hence,
so.addVariable("breadcrumb", "[[breadcrumbs]]");
should probably be:
so.addVariable("breadcrumb", "[[Breadcrumbs]]");
The Breadcrumbs snippet is not lowercase, at least not in 0.9.6.3 with the default install.
But I agree, if you want to import entire HTML parts (menus, pages etc.), it’s probably safer to use XML with Ditto and custom template-chunks. If you expect HTML in a snippet output, make sure you use CDATA in your XML nodes.
http://www.w3schools.com/XML/xml_cdata.asp
btw, you can debug your Flash movie if you list all variables in the authoring environment. Perhaps there are special characters in the snippet output that cut off part of the variable (line-breaks, ", ’ = etc.).
If you have lots of server-Flash interaction, maybe using something like AMFPHP is even better (and faster) than using XML:
http://www.amfphp.org/